Assorted fixes required for CRAN submission

[rpkyle]

This PR proposes several enhancements and minor fixes in the R package generator, based on requests from CRAN maintainers:

• support for arbitrary file extensions in component library assets (required by Provide support for arbitrary file extensions within Dash for R component libraries dashR#186) superseded by Issue 481 - Support arbitrary file extensions in component suites #1078
• fixes for wildcard props handling (ensures these are generated within components)
• adjust line wrapping in R online help to satisfy CRAN upload requirements (wrap at 60 chars instead of 80)
• add all authors and copyright holders to Authors@R field within DESCRIPTION
• add \value field to all .Rd files for exported methods and "explain function results". for functions returning no value, document as \value{None}.
• where copyrights held by an entity other than package authors, indicate via cph role attributes in the Authors@R field, or by using a Copyright field.

Specifically, two new YAML keys are supported in dash-info.yaml:

• pkg_copyright: when present, will add in a Copyright: line including the string value (see https://cran.r-project.org/web/packages/policies.html)
• pkg_authors: when present, dash-generate-components will use this string as-is for Authors@R:, which CRAN prefers to Author: and Maintainer: elements within DESCRIPTION. When this string is not present, the generator will create it based on the author key within package.json, and assume that the author is the maintainer.

[Marc-Andre-Rivet]

@rpkyle I think we should drop _asset_dist and just use _js_dist for now. While the _asset_dist is an interesting addition, I think we should rework css/js/asset in depth and with all Dash flavors in mind before committing to supporting additional props. _js_dist is not perfect but it covers all usages with _css_dist

This PR Portions of this PR would be superseded by #1078.

[rpkyle]

@alexcjohnson @Marc-Andre-Rivet This is ready for your review, and should resolve the CRAN maintainer concerns. I'd like to merge this today if possible, so Marc-André can use it for package builds starting with the upcoming Dash release. 🙏

[alexcjohnson]

💄 There are a bunch of places here the indentation is just changed from 4 to 8 spaces inside parens, either function args or if conditions - please put back the original 4 spaces. Also there's a ._r_components_generation.py.swp file committed -> should be .gitignore'd?

Aside from that these changes look good to me!

[rpkyle]

💄 There are a bunch of places here the indentation is just changed from 4 to 8 spaces inside parens, either function args or if conditions - please put back the original 4 spaces. Also there's a ._r_components_generation.py.swp file committed -> should be .gitignore'd?

Aside from that these changes look good to me!

Sure, not entirely understanding what happened there, but the linter demanded more spaces, so I added them. 🤷‍♂️ I can try removing them, but if it fails the check I'll have to determine what led to the (new) warnings.

Will add the vim-generated swapfile, should probably .gitignore all .swp files, never any reason to commit them.

[rpkyle]

Also there's a ._r_components_generation.py.swp file committed -> should be .gitignore'd?

fixed in ae28bac and bc903bc

[rpkyle]

💄 There are a bunch of places here the indentation is just changed from 4 to 8 spaces inside parens, either function args or if conditions - please put back the original 4 spaces.

OK, restored in 06dbc8d. Will try to sort out why that happened, might be a difference in configuration between pylint on my machine vs. CircleCI.

[alexcjohnson]

This PR should be retargeted to dev now that #1078 is merged, yes? Anyway after that and the Windows build is sorted out I think it's ready to go! 💃

[rpkyle]

This PR should be retargeted to dev now that #1078 is merged, yes? Anyway after that and the Windows build is sorted out I think it's ready to go! 💃

@alexcjohnson I've taken your advice (thanks! 👏 ) on removing the spaces within the --r-suggests statement inside of package.json in plotly/dash-core-components, and the workaround enables the test to pass, but we still haven't sorted out why it fails (and we should).

I've opened https://github.com/plotly/dash-core/issues/184 in the meantime, and 019df65 will add back the spaces we have now omitted when necessary.