Added Devise_invitable gem.

This will make the app invitation-only while in beta stage.
WIP: make the application invitation-only, by integrating devise_invitable gem.

This will stay at least during the beta stage. Afterwards it would be nice if people could invite other people, but the invitation-only restriction will be lifted.
Generated devise_invitable views for customization.
Customized devise_invitable routes.
Updates aws-sdk gem from 1.42.0 to 1.43.1
Minor reorganization of routes for better readability.
Only admin users are currently allowed to send invitations.
Added has_many relationship to User model, to know who invited whom.
WIP: users can send invitations via email to join Feedbunch.
Minor change in texts.
Minor cosmetic changes.
Organize panels in the edit registration view in columns, for sm- and bigger screens.
Give "send invitation" button an id.

Useful mostly for testing.
Finished styling the "invite a friend" popup.
WIP: users can send invitations via email to join feedbunch.
WIP: user invitations.
Controller test now sends the correct params.

It's still failing a bit further along the controller logic. When I get it to pass I'll carry this change to the angularjs service that actually makes the POST.
Users can send invitation emails.

Implemented: email-sending

Not yet implemented: Validate that the user does not exist. Validate that the email is valid. Accept invitations.
Use current_inviter instead of current_user for more homogeneous code.
Finish the change started in e9f960b72b56e72844fd27bab412a6f77c9a4ce8