feat: add label links

[davwheat]

Fixes #31

Changes proposed in this pull request:

Adds the ability to create 'label links' (links without URLs) for separating a larger dropdown, or as a dropdown button itself.

Screenshot

https://user-images.githubusercontent.com/7406822/137226835-3c3b09f2-c857-4164-8e05-b9a904203d6c.mp4

Confirmed

• [x] Frontend changes: tested on a local Flarum installation.
• [x] Backend changes.

[imorland]

Just tested this out locally (see my dev site too)

There's some colouring issue on the blank link, and also on the various :hover states for each type :'(

See video sent on Discord