Different permissions for administrators in the admin panel

## Feature Request
It would be nice to have different levels of access to the admin panel. E.g. one group could modify the settings of a specific extension, but could not disable / enable it. Another group could edit users, etc.

**Is your feature request related to a problem? Please describe.**
This is useful on large forums where there are a lot of people on staff.
Just our forum has this problem, as we are migrating a large forum from Invision Community to Flarum. We have admins who deal with user issues, admins who can change forum settings and technical admins who can change more advanced options.

**Describe the solution you'd like**
Currently Flarum permissions only apply to the forum application. These should be extended to include the admin application as well.

**Justify why this feature belongs in Flarum's core, rather than in a third-party extension**
I think complete forum software should include a complete permissions system. Other forum softwares have this.
Such permissions could then be implemented by other extensions so that, for example, only specific groups of administrators could change specific settings.