Get rid of Laravel Gate contract

[askvortsov1]

**Refs #2055 #2092 **

Changes proposed in this pull request:
As of right now, the only piece of the gate we actually use is the before callback in UserServiceProvider. This PR replaces the gate with that callback, freeing us from the Laravel Gate contract that we don't use anyway. Unless I made a mistake somewhere, no functionality has actually been changed.

Next steps:

• Instances of models should not matter when checking permissions #2186
• Implement the extender-able, namespaced policy system discussed in Refactoring Policies #2092.

Reviewers should focus on:

• What here, if anything, do we need to provide a BC layer for? $user->can has been around for a while now, and Im assuming that is being used instead of directly calling the gate.

Confirmed

• Frontend changes: tested on a local Flarum installation.
• Backend changes: tests are green (run composer test).

[franzliedke]

Please rebase!

The changes from #2186 now need to be applied in the new Gate class, not in the UserServiceProvider, I believe.

[franzliedke]

Code looks good!