Add initial support for username/community mentions

[micahmo]

Pull Request Description

This is a first-pass implementation for adding easier username and community mentions in the comment and post editor. For this first pass, mentions can be initiated via editor buttons and a popup dialog.

In the future, it would be nice to detect that a username/community is being mentioned while typing (@/!) and show relevant results inline based on what has been typed (like the web UI). That will be a bit more work, so I figured we could get the groundwork in place here (and I also did some refactoring to that end).

Note that thunder-app/markdown-editor#3 must be merged first, then I'll update pubspec.yaml here.

Review without whitespace.

Issue Being Fixed

Issue Number: Related to #256 and #500, but not a complete solution.

Screenshots / Recordings

qemu-system-x86_64_7iDUMAi5QD.mp4

Checklist

• Did you update CHANGELOG.md?
• Did you use localized strings where applicable?
• Did you add semanticLabels where applicable for accessibility?

[micahmo]

Updated to use the merged package ref from the official repo, so this one should be good to go!

[hjiangsu]

LGTM!

This isn't directed towards this PR in particular but I'm just keeping a mental note to refactor/extract the MarkdownButtons into its own separate widget to have less repeated code in the codebase!