Editor: Make Fediverse preview template filterable

[obenland]

Fixes #967.

Proposed changes:

• Adds a filter for the preview template path.
• Adds unit tests for the filter.

Other information:

• Have you written new tests for your changes, if applicable?

Testing instructions:

• Go to the Editor to create or edit a post.
• Click Preview > Fediverse Preview.
• Make sure the preview still works.

[pfefferle]

I think this is a nice first step, but I would keep the ticket open, to discuss a template manager, that allows you to view your post in different templates. Or should we file a different issue for that?

/cc @Menrath