Remove sourcehut-migrate

It now lives in its own repo [1], so it can be tagged and packaged
independently of core-go.

[1] https://git.sr.ht/~sircmpwn/sourcehut-migrate

Add sourcehut-migrate, a unified DB migration tool

This tool will replace both the old `srht-migrate` as well as all
`<SERVICE>-migrate` tools, but services need to be migrated over to it
one by one. This tool is based on brant [1].

Invocation in both production and development can be as simple as:

    sourcehut-migrate <SERVICE> up

The tool will read a config like a regular service (first in ., then
/etc/sr.ht, etc.). The database connection details are taken from the
config. To facilitate development, the tool will check if a folder
./migrations exists; if it does, migrations will be loaded from there.
If no ./migrations folder was found, it will look for migrations in
`<ASSETS>/migrations/<SERVICE>`, where <ASSETS> is also taken from the
config, with a default of `/usr/share/sourcehut`.

For an explanation of the available commands, see brant [1].

This tool also supports a simple command to initialize a newly created
database to the current version (much like the `<SERVICE>-initdb` tools
used to):

    sourcehut-migrate <SERVICE> stamp head

will automatically set the database version to whatever the latest
available migration version is.

[1]: https://git.sr.ht/~bitfehler/brant

go fmt

auth: s/OAuth2Token/BearerToken/g

This also makes some tweaks to the organization of the crypto module to
make it easier for us to add more keys in the future, which will be
necessary for the internal token redesign.

Add debugging command to decode OAuth tokens