To provide a means for users of DMD to better understand whether a query
that they wrote a few weeks ago will work with the latest version of the
tooling, without poring through the release notes (or the slightly
easier-to-scan Slack notifications), is the use of a new field in the
`metadata` table.

This provides a new known field in the `metadata`, which allows for
understanding the _minimum_ version of the `dmd`, `dmd-web` or
`dmd-graph` tooling that can be used with a given database.

We pin this to v0.86.0 as that was the last time a breaking change was
introduced, and any commands or queries that were performed with the
tools from that version of DMD should still work.

This requires a rename of the `metadata` variable in `db_init.go`, so we
can import the package more easily.

We can make sure that the `CompatibleSince` version is exported as part
of our public API to make it something others can rely on
programatically, too.

As part of #532.