[APP-870] Compose intent URL

[pfrazee]

Add a URL for triggering the composer with some pre-filled fields. This is useful for websites that want to do share links/buttons.

See bluesky-social/atproto#1671

_APP-870

[ShreyanJain9]

When you do this, I would suggest figuring out some way to make it client-agnostic, if possible. Might not be easy, but I think you can do it!

[pfrazee]

Copying from dup:

Is your feature request related to a problem? Please describe.

Currently users wanting to share off-site links directly from the page need to do a variety of URL copy/paste actions to get a link into the Bluesky editing interface. Further, offsite content creators who want to "pre-fill" the sharing post's text are unable to do so. This makes it impossible to create off-site sharing buttons, such as you'd have for every other social media.

Describe the solution you'd like

Two APIs:

• /share?url=<url>&prefill=<prefill text>&origin=<handle of account linked to shared content>
  • User journey: When a user clicks a share button outside of Bluesky, she is taken to a new post dialog. If the optional prefill query parameter has a value, that text precedes the string given via the required url query parameter. If the optional origin query parameter has a value, a translated version of a string in the format ", via @{handle}." is appended to the text.
• /repost?did=<did>&prefill=<prefill text>[&allow_quote]
  • User journey: A user clicks a link in this format on an off-site page, which opens the related DID in the Bluesky interface, possibly without any attached replies. If the originating website included the optional allow_quote query parameter, the "Repost"/"Quote post" menu is already unfurled, allowing the user to select either option. If "Quote post" is selected by the user, the quote post editor is opened; if the prefill query parameter is included, the editor is already populated with the text from that. If the "enable_quote" query parameter is omitted, the user is given the option to repost the linked DID without a quote. Of course if the user really wants to quote post a link, she's still able to copy/paste the URL into a new post.

Describe alternatives you've considered

Prior art:

• X: /intent/tweet?url=<url>&text=<prefill>&via=<handle>
• Facebook: /sharer.php?u=<url>&t=<prefill>
• LinkedIn: /sharing/share-offsite/?url=<url>

Of these, X's implementation fulfils the needs of media orgs the best by allowing both prefill text as well as the ability for a shared post be linked to the org's canonical user.

Additional context

I'm a senior developer with Financial Times and need at least the first API I suggested to be able to add "Share on Bluesky" buttons, which I highly suspect we'll want to do this year.

Might be in some ways related to #66.