User Accounts: Add flask login capability

[nknaian]

Literally just follow https://blog.miguelgrinberg.com/post/the-flask-mega-tutorial-part-v-user-logins ...seems like it's all there

At the end of this there should be:

• the ability for people to login/logout by pressing a button
• there should be a user page with some really basic info, like showing their username etc.
• login shouldn't yet be required for anything...

[nknaian]

Instead of logging in using a password, spotify can be used for authentication ('log in with spotify')

[nknaian]

Closing this for now in favor of #26....the capabilities that Flask-Login gives aren't necessary if we're using spotify as login authenticator