Quick message module

Suggested in #854.
* [Looks like this](https://i.imgur.com/1Y40NAS.png)
* Default keyboard shortcut "ctrl-m".
* Launch from command line with `qm [recipient]`
* Option to open this when clicking on "send message" in user info hover (enabled by default).
* And when clicking on "message the mods" (disabled by default).
* I didn't forget about nightmode this time! (almost did)

No option for reporting, unfortunately, because I couldn't get it to work (and it might not be as useful now that we have report reasons).