Create a new repository as if created with 'init --shared'

This change will adjust a new repository to appear like it has been created with the '--shared' option under a unix-like OS. The respective configuration entries are set and (for Unices) the file permissions are adjusted. 
This is useful for scenarios where Gitblit is installed as a management front-end to Git repositories but actual Git access is provided by different means, e.g. over SSH and HTTP. In that case creating repositories as shared helps keeping the file permissions right to maintain shared access to the repositories.

One caveat: when running Gitblit under Linux with an effective group id different from the group id that the repositories are created as, e.g. because the base folder for the repositories is already set-gid with a different group, file permissions are not adjusted. This is because under Linux this would clear the set-gid flag under these circumstances. In such a case correct file permissions have to be achieved by choosing an appropriate umask for the Gitblit process.

Currently this is implemented with a configuration option git.createRepositoriesShared instead of a per-repository option in the edit page. It seems that setting this option is more related to the installation scenario. On the other hand it could make sense to be able to switch it on or off for personal repositories but so far I did not anticipate the need and therefore left is as a global configuration option.