Set up HTMX and add button to subscribe to leagues

[jeddai]

As it is now, this accomplishes part of #11, namely adding a button for users to subscribe to leagues and updating the database/UI accordingly. Full list of things changed here:

• Unique constraint on User and League added to Subscription model
• HTMX enabled in Django configs and base HTML
• New function-based "view" and url added for toggling subscribed state
  • This view returns only the innerHTML content that should be replaced in the "Subscribe" button
• Template tag filter for checking League subscription status

The remaining part is sending emails to subscribers, though I'm not entirely sure how to go about doing that based on the current structure of the code. @davidmreed if you have any guidance there it would be greatly appreciated!

[davidmreed]

With apologies for the conflict: I'd like to apply the HTMX feature on top of #192

[jeddai]

By all means!