The textarea in the CustomFooterModal was much larger than in the other appearance page modals, and did not use a monospaced font. Turns out the other 2 were explicitly specified in the less. This commit adds a class that can be applied to all these modals for simpler maintenance.
Fixes https://github.com/flarum/core/issues/2865
- Extensions now have their own pages
- The API for extensions to register permissions and settings has been overhauled via the `flarum/admin/utils/ExtensionData` util
- An extension grid has been added as a widget to the Dashboard page
Closes#268. Not going to bother with a preview SVG or anything fancy for now – we can think about that as part of #746. Right now it's just good to finally get this functionality in!
Also need to think about apple-touch-icon, msTile stuff, and social sharing image. Not sure if this is all too much for core, but it's definitely too much for the current Appearance page layout. Again, something to think about as part of #746.
Code is a bit rough around the edges, but figured there's not much point in using the command bus properly since #870.
