Introduce support for draft PRs

[Fs00]

This PR makes OctoDroid able to recognize draft PRs and update the UI accordingly.
When a PR is a draft:

• the UI color changes to gray for the app bar and comment/review sheets' bottom bar. The base gray color (#6e7781) was taken directly from the GitHub web UI, all other tones were derived from that
• the merge button disappears since draft PRs can't be merged

The app now also displays converted to draft / marked as ready for review events in PR conversation.

Extra notes:

• I've removed pressed states for edit issue/PR fab in cd22140 because they are actually useless thanks to ripple. They were also visually conflicting with the latter.
• in 771318e I've merged two drawables for review-related PR events that were perfectly equal. I don't see a good reason to keep them separate, given also that other PR events share drawables with others too

Fixes #1040

[Fs00]

A sidenote: I would have liked to implement a way to distinguish draft PRs in the open PRs list.
However, this requires a non-trivial amount of work since issue and PR lists share the same adapter which operates on Issue objects. Issues have a pullRequest field which holds a partial PR object with almost all fields (including draft state) set to null.
Therefore I doubt that the work required to implement this distinction is worth it.

[maniac103]

Therefore I doubt that the work required to implement this distinction is worth it.

I agree. It's also really sad there's no API for converting a PR between draft and normal state.