Create a dedicated package `.../pkg/website` to contain constants for the
URLs of documentation, bug reporting, etc. That way it's easier to see
which parts of the website are being referred to from the Flamenco
binaries, and updates can happen in a central spot.
No functional changes.
Convert the database interface from the stdlib `database/sql` package to
the GORM object relational mapper.
GORM is also used by the Manager, and thus with this change both Worker
and Manager have a uniform way of accessing their databases.
