Request to join group

This pull request addresses #458
Open policy does conflicts currently with our [permissions specification](https://github.com/peerlibrary/peerlibrary/wiki/Permissions#modifying-permissions-group-membership). There is written that only administrators can modify group membership. The rationale is that groups can be used for permissions elsewhere in the system, so we are a bit more strict here. I agree that having a concept of open groups is an interesting one, but then we have to modify specification accordingly. Probably splitting "Modifying permissions" and "Group membership" into two sections, keeping that permissions can only admins modify, but membership depends on the policy.

BTW, if policy is open and group itself is added to group admins, then anyone has admin permissions over a group itself. :-) I would allow to do that, but this can happen once you allow open membership.