Add a uid field in the Issue object storing a unique identifier for all the tickets

This uid field is used as name for the file in git storing the information
about that ticket in git
Add the update_git_ticket to the internal API

This method forks a given project and update the JSON representation of
the specified issue. The file containing the JSON is named after the UID
of the issue stored in the database.
Upon additions or editions in the git, changes are pushed back to the
main repo
Update the git for tickets upon creation, edition or addition of new comments
Fix bugs in the issues mapping so that is always relies on the global identifier
Fix giving a unique identifier to each ticket upon creation
Fix updating the git repo for tickets when editing an issue
Fix the git integration of the issues, we return if there are not files and not addition