[WIP] Allow users to request access to projects.

Created by: cirosantilli

Fix http://feedback.gitlab.com/forums/176466-general/suggestions/6201690-request-access-procedure

User requests access from the project's main page:

Project admins get an email linking to the moderation page, where it is possible to either accept or decline them one by one, or accept all at once:

All access requests are initially set to the same access level: I propose Developper as it is a common use case. Guest would be too little since you need it or equivalent (e.g. public project) to see the project and make the request in the first place. In another PR we could also allow the requester to determine the level he wishes to request if people feel the need.