To separate the different kinds of repositories we have at GitLab this
table will be renamed to pool_repositories. A project can, for now at
least, be member of none, or one of these. The table will get additional
columns in a later merge request where more logic is implemented for the
model.

Further included is a small refactor of logic around hashing ids for the
disk_path, mainly to ensure a previous implementation is reusable.

The disk_path for the pool_repositories table no longer has a NOT NULL
constraint, but given the hashing of the ID requires the DB to assign
the record an ID, an after_create hook is used to update the value.

A related MR is:
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/23143, adding
tables for 'normal' repositories and wiki_repositories.