[RFE] Rework URL structure

[g2p]

I would like pinafore URLs to have a structure that is more predictable, useful for typing or for tools.

For example, the URL structure that works in Mastodon instances:

• https://fosstodon.org/@pinafore
• https://fosstodon.org/@pinafore/109418903520996488
• https://fosstodon.org/@[email protected]

Becomes (assuming the default URL for pinafore and fosstodon as your home instance)

• https://pinafore.social/@pinafore
• https://pinafore.social/@pinafore/109418903520996488
• https://pinafore.social/@[email protected]
  (abridged if hosted on the home instance, non-abridged otherwise)

As far as I understand pinafore's multi-instance/multi-account support, only one is active at a time, so they don't need to be reflected in the URL. (If multiple can be active, non-default instances could use a prefix)

This would become immediately useful with this small extension for interacting with remote profiles: https://github.com/bramus/mastodon-profile-redirect/