ProtectedBranches model, Master permission for repo\n Allow push to protected branch for masters only

Project activities perfomance increased. Cache for project activitites disabled. Repository conception changed

This commit includes:

 * Projects can have zero or more WebHooks.
 * The PostReceive job will ask a project to execute any web hooks defined for that project.
 * WebHook has a URL, we post Github-compatible JSON to that URL.
 * Failure to execute a WebHook will be silently ignored.