Correctly handle serving BookWyrm on custom port

[Minnozz]

• [x] Include correct protocol and port in remote IDs
• [x] Store correct protocol and port in BookwyrmConnector
• [x] Construct absolute URLs with correct protocol and port
• [x] Test custom port by running pytest with it

[Minnozz]

~Hmm, CI itself is broken, unrelated to my change (probably because CI always uses the latest version of eslint instead of the one in dev-tools)~

[Minnozz]

@bookwyrm-social/code-review this PR is ready for review

[mouse-reeve]

Thank you for the housekeeping work along with this change!