Add user membership management to group management

[kbroschke]

closes #106

[KiruChaff]

We've tried implementing the feature through a form which is supposed to call the add_user method in the group controller. Unfortunately this method is not called through.

Maybe validate the resource and check if the form is correct

[TobiPeterG]

Unfortunately, I don't have a second user account to test this feature; is there any easy way to add a new user to the database?

[DieKautz]

Whats still missing is

• handling if the specified email belongs to no existing user and
• shrinking the class length of the groups controller
  membership management maybe extracted to its own controller?

[kbroschke]

I implemented the idea of moving the user management logic into a separate controller. But there is still duplicated code left, because sharing code between controllers seems to be non-trivial. I already looked into using Concerns. Before using a separate controller, I also experimented with other solutions to our problem. But in the end, just pointing the routes to the new MembershipsController was the easiest one to implement.

[TobiPeterG]

When a user is not found, an alert should be shown instead of a notice. I added that.
Also there was an unused function added in the GroupsController, I removed it.
Might be useful for a refactor, but not this issue here.
Otherwise, it looks good!

[TobiPeterG]

If someone could quickly look over my changes, we're good to merge!

[kbroschke]

I can't explicitly approve my own pr, but everything looks good to me. At least with refactorings tracked in #346 and in #353.
Maybe at least one additional person should review all changes (@KiruChaff or @DieKautz?) because Tobias and I made so many recent changes.