Add an option to always show full usernames

**Is your feature request related to a problem? Please describe.**
At the moment, Phanpy shows shortened usernames in its UI - only the local username (as Mastodon docs call it) is used, not the instance domain.

I would prefer to see full usernames all the time.

One use case for this is sometimes I screenshot posts for sharing, but when doing so, the post author and any mentioned users cannot be easily found their instances are not visible.

**Describe the solution you'd like**
A toggle within Settings to allow opting-in to showing full usernames in the feed and in a post preview, instead of shortened names.

**Describe alternatives you've considered**
Using an alternative front-end or writing some kind of userscript to re-instate full usernames.

**Additional context**
I'm willing to contribute code towards this, just want to get a consensus before I actually spend time on it.