Use different DB and session store backend

[sickelap]

Describe the enhancement you'd like
Allow users to choose which database and/or session store backend to use. For example SQLite + file-based session store.

Describe why this will benefit the LibrePhotos
In addition to providing a choice, I think this will benefit other use cases such as described in #489 or LibrePhotos/librephotos-linux#44. Fewer external dependencies will also simplify the initial setup. Of course, it will likely increase maintenance overhead but before judging whether this is the case I'd like to propose trying to switch one thing and see how it goes. This could be either adding support for another DB backend or a different kind of session store.

[webysther]

sqlite will be nice

[avibrazil]

SQLite would be great for simpler setups.
Please consider using SQLAlchemy for database abstraction, so users can choose what they already have running.