Rework admin-fe authentication
The authentication used in admin-fe right now is the password grant
OAuth login flow. This is not compatible with Pleroma instances that have custom auth backends. We should be using the authorization code grant
which is how mobile apps and other 3rd party apps do authorization. It's also more secure.
https://www.alexbilbie.com/guide-to-oauth-2-grants/
http://tools.ietf.org/html/rfc6749#section-4.1
I'm also not against the idea of having the ability to specify the server to login to so one hosted admin-fe can be used to login to any public instance you have admin credentials for. There are definitely benefits to that approach for multiple reasons and can make testing against different Pleroma instances easier.