Make sure Python Wheel contains all that's needed

[scy]

The Wheel we're building as part of the GitHub Actions CI run should include everything required to run DearMEP. In other words:

• the backend
• the frontend
• default configs
• database migrations

Not included are:

• dependencies (i.e. other Python packages that can be installed via the package manager)
• a list of MEPs (third-party users should select these themselves)
• name audio & MEP portraits (we don't have permission to redistribute them, third-party users need to download these themselves)
• IVR audio and group logos (should be distributed in some other way, see Provide IVR audio & group logos #272)

I'd especially like to know if the database migrations work out of the box.

[scy]

The migrations weren't even included in the Wheel, and neither was the Alembic configuration. Added both in #304 and made it way easier to use.

[scy]

Wheel is looking good now, did a fresh setup based on it.