Refactor manuscript format text scaling

[vkbo]

Summary:

This PR fixes:

• The DocX line height setting, which was correct in Libre Office, but not in Office 365. It does now scale to base font size, with an auto scaling for headers. This works in both office apps.
• All ODT margins now scale to font size rather than font size and line height.
• All HTML margins also scale directly to em.
• Update default margin values so they produce roughly the same result as before.
• Fix DocX page count so that it works in Office 365.
• use icons for text margin and page size settings instead of single character labels.

Related Issue(s):

Closes #2098
Closes #2099

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