Add external URLs to apps

[freddyheppell]

Closes #686

This PR adds a new "External URL" field to each app, which allows you to specify a different URL that is externally routable. If specified, this is used to generate clickable links in the frontend, instead of the API URL. This is necessary when you're using a reverse proxy because the API URL will be the app container name (e.g. http://sonarr:8989) behind a reverse proxy to a publicly routable URL (e.g. https://sonarr.example.org). This mirrors existing functionality in *arr-adjacent software, e.g. Overseerr.

Changes:

• Added a new external_url property to each app's default config
• Added a new config migration feature where missing instance-level keys are set to their defaults
• Modified the forms for each app to include the field
• Modified the link generation code in the hunt manager and history to use the external URL if set

I have tested that an old config database (generated by the current main branch), is succesfully migrated to include the new key, and that the forms work. I haven't been able to test that the frontend link generation actually works (because my dev instance isn't actually connected to any *arrs), but the code is pretty straightforward.

[Admin9705]

@freddyheppell this looks promising. i'll incorporate this into 9.1 - working some major overhauls. thanks for this!