Fix compose button on mobile

[davwheat]

Fixes #155

Changes proposed in this pull request: Adds missing class to compose button to make it correctly show on the app bar on mobile.

Confirmed

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

Required changes:

• [ ] Related Flarum core extension PR's: (Omit this section if irrelevant)

[davwheat]

woahhh what happened there

get out of there sneaky commit