add mara_db click group

[leo-schick]

This adds the commands via a click group. In the future, cli commands should be run via

flask mara-db migrate

instead of

flask mara_db.migrate

Using the old syntax produces a warning that the old style will be dropped in version 5.0.

In the future, a new package with an entrypoint mara will trim leading mara- from the click group name. Then it would be possible to run e.g.

mara db migrate

But to avoid complications with other packages (see discussion in mara/mara-app#44), we use the module name as click group name.