ActivityPub protocol scheme (web+ap link) support

[TheEssem]

Pitch

There should be a way to handle direct links to a post or user without having to go through the user's home instance, in such a way that the link directly opens the web client of your instance. This can be done using web protocol handlers for web frontends and through other methods for native apps. As of right now, there's no standardized way to handle this; however, some implementations and the Fedi Links project have settled on using the web+ap scheme. Other potential schemes worth looking into could be fedi, apub, or activitypub (or any of these with web+ appended to the beginning). See the following links for further discussion and info regarding this:

• https://akkoma.dev/AkkomaGang/akkoma/pulls/589
• https://github.com/mastodon/mastodon/issues/19679
• https://github.com/activitypub-schema/proposal/blob/main/PROPOSAL.md
• https://codeberg.org/fediverse/fediverse-ideas/issues/1
• https://fedilinks.org/
• https://firefish.dev/firefish/firefish/-/issues/10756

This commit could be a potential base for the implementation: https://github.com/arachnist/mastodon/commit/161a19f3aa8d9fa40ae2a08fdb81e42939ae32dd

Motivation

This could potentially reduce confusion with new users regarding posts on other instances, especially when those instances are using different software.