Populate admin with users

[msvrtan]

We should administer users via admin

[HcsOmot]

If we can somehow use easyadmin to create new users (fos:user), maybe that could help closing #169 and #163)

[HcsOmot]

We can create a new user record using easyadmin, but the newly created user cannot log in.
Easyadmin writes a plain text password directly into the db, bypassing fos:user password hashing.

Proposed solution:

• create a custom registration form for creating new admin users
• use fos:usermanager to create the new user with provided credentials
• protect the route
• add menu item to easyadmin backend

Ping @msvrtan for comment.