Add basic user interface

[Sporiff]

The Open Podcast API server is primarily headless. However, a frontend is needed for simple things like displaying a login form/OAuth permission form. In addition, we may want to display some simple information to users in a web interface for testing purposes and eventual deployment purposes such as:

1. A list of subscriptions.
2. Information about specific subscriptions.
3. Actions on subscriptions (e.g. unsubscribe, subscribe)
4. Popular subscriptions on the server.
5. User settings (e.g. update password, change username, update email address, delete account).

This is dependent on implementing and extending #2. Currently, the server supports authentication using Spring Security and form-based login. This is good enough for login, but we will need to style the template to fit with the rest of the interface. We will also need to implement a better password encoder (see Spring Security docs about DelegatingPasswordEncoder).

This user interface SHOULD work without JavaScript and MAY be progressively enhanced using HTMX.