Rather than expecting it always to be "guix", store the expected value in the
database, and use the value of the header to find the relevant repository.
The git_branches table had 'NULL' values for some commits where the branch was
deleted, importantly this was the string 'NULL', not an actual NULL value.
This commit fixes that, migrating the existing values to be '', and changing
the relevant code.
The primary key is also extended to include the datetime field, as this is
important to allow a branch to be deleted twice.
Add some new pages /branches and /branch/... as well as a new git_branches
table. Also extend the email processing to enter the branch information in to
the database.
Rather than just storing the URL in the guix_revisions and
load_new_guix_revision_jobs tables. This will help when storing more
information like tags and branches in the future.
