Add flask-migrate to enable database migration

[mschader]

This will add functionability to create database migrations. The concept was taken from:
https://blog.miguelgrinberg.com/post/the-flask-mega-tutorial-part-iv-database

[fleaz]

So hyped to get DB migrations. No more manual fiddeling around in the prod DB via cli :D

Will this happen automatically or do we have to write migration scripts?

[mschader]

As far as i understood, these changes will be sufficient to initially crate the migrations structural folder and upgrade the database by simply using flask db <command> (e.g. flask db upgrade)

You are even able to only select single tables to migrate and stuuff like moving forward/backward in versions.

[fleaz]

The failing TravisCI tests are a bug in pipenv--> #3026

[mschader]

I would suggest to merge this branch already into development so we can start using it as soon as #19 is finished.

[fleaz]

Made a squash merge because this feature branch contained strange commits because the branch was created from master instead of development.

Thanks!