__init__: minor import cleanup

refs #15831
__init__: remove db init

refs #15831
__init__: pep8

refs #15831
models: cleanup imports

refs #15831
models: use declarative model, no Flask-SQLAlchemy

refs #15831
database: use the app, not current_app

refs #15831
api: new database changes

refs #15831
util: new database changes

refs #15831
general: new database changes

refs #15831
category: new database changes

refs #15831
util: import cleanup

refs #15831
category: added string_to_type to imports

refs #15831
general: added constants to imports

refs #15831
requirements: updated requirements, removed Flask-SQLAlchemy

refs #15831
Replace hardcoded path with a config option

refs #15879
Merge branch 'enhancement/15831' into develop
Merge branch 'bug/15879' into develop
initdb: command added to manage.py

refs #15699
initdb: added info in the readme

refs #15699
models: added student integer to sites_courses

refs #12753