Add documentation, tests, and cleanup stylesheets

[sssoleileraaa]

Description

The following issues around stylesheets in Qt could be avoided if we have documentation on how to set stylesheets in the client, remove "bad" examples from the code, and add stylesheet tests to our manual test plan or test suite.

• upstream stylesheet bug causing segfault, see segfault when clicking on Source #254 (comment) for context
• switching out CSS for different reply states
• switching out CSS for different message states
• segfault when making QLabel selectable, see 22e1b34 for our solution

[sssoleileraaa]

• segfault when making QLabel selectable, see 22e1b34 for our solution

I haven't had much time to look into this issue but here's an interesting finding:

• this segfaults
• this does not segfault

More to come next week!

[eloquence]

Deferring to a future sprint in favor of closing out the other docs task we already have a PR for, #1089

[sssoleileraaa]

Add documentation, tests, and cleanup stylesheets

The bulk of this work to create tests and cleanup/refactor how we style the gui happened more than a month ago, so I went ahead and started a wiki page around project organization so that we can close this out.