Add an option to change mentors into the "godmother-only" role

[famaniel]

As with resolving issue 17, the status of being godmother is independent from a user's role (although mentees cannot be godmothers). If a person who is a god mother cannot attend congress, or for any other reason, also be a mentor, they can be made godmother only. A person who is in the godmother-only role has to be a godmother. Therefore the UI needs to provide the following

• A control that allows the choice between mentor and godmother
• The control must only be available to users who are godmothers
• The control can only be set, if someone is a godmother. If the godmother status is revoked, the user falls back to being a mentor.

[famaniel]

The roles are now changed in a way, that the system behaves in the following way:

• Godmothers can change the role of a person
• If you select someones role to be mentee, their godmother status will automatically be revoked.
• if you select someones role to be mentor, their godmother status can be set by another godmother
• if you select someones role to be godmother-only, the godmother status will be set.
• Godmothers cannot revoke their own godmother status, to prevent the situation that all godmothers revoke their status, and thus effectively lock everyone out.

There is still an issue when displaying the status before saving. To hide the "is Godmother"-flag, some Java-Script is needed, because rails cannot modify any interactive elements once they are displayed, without reloading the page (at least, I didn't find out how).

[famaniel]

Merged and closed the issue with master branch