Make database migrations an explicit action

[Terrance]

I vaguely remembered reading about the database migration in the release notes, though I only today needed to reach for beet and was surprised that calling --help started a migration:

$ beet import --help
Migrating genres for 10144 items...
  Migrated 1000 items (1000/10144 processed)...
  Migrated 2000 items (2000/10144 processed)...
  Migrated 3000 items (3000/10144 processed)...
  ...

This isn't ideal when you're connected to a remote server over a flaky connection! It's unclear what would happen if interrupted -- with the progress reporting I'm assuming this wasn't an atomic operation. It took about 4-5 minutes to run in the end, after which I got the help I requested:

...
Migrating artpath for 1138 albums...
Migration complete: 1138 of 1138 albums updated
Usage: beet import [options]

Options:
  -h, --help            show this help message and exit
  ...

Ultimately this was a one and done, but for future migrations it would be nice to make running the migration an explicit action, e.g.

$ beet import
Database migration required.  Please run `beet migrate`.
<exit code 1>

$ beet migrate
Migrating genres for 10144 items...
...

...or at least exclude no-op commands like --help from starting it.