Expose text margin settings in Manuscript Build

[vkbo]

Summary:

The text margin settings have always been there internally, but this PR adds them to the Build Settings form, and also makes the margin settings layout more compact.

In addition, this PR cleans up the code so that text sizes and margins are defined globally in constants.

Related Issue(s):

Related #2023

Reviewer's Checklist:

• The header of all files contain a reference to the repository license
• The overall test coverage is increased or remains the same as before
• All tests are passing
• All flake8 checks are passing and the style guide is followed
• Documentation (as docstrings) is complete and understandable
• Only files that have been actively changed are committed