Squash old migrations, simplify Vagrant instructions

[cmfcmf]

This squashes all migrations older than 2018 into a single big migration using Squasher. It also removes mentions of Postgres from the README, which caused multiple issues:

• slow test execution
• unrelated changes to schema.rb (@corinnaj)
• fixes Migrations with sqlite throw error #698

I also simplified the steps to take once you run vagrant up for the first time.

[coveralls]

Coverage remained the same at 69.164% when pulling 3465d31 on cmfcmf:squash-migrations into 711d18e on hpi-schuelerklub:dev.

[corinnaj]

Great idea! Looks good as far as I can tell...