Improve support for finding same post on alternative instance

[brainchild0]

Problem

Suppose a user browses to a post through a foreign instance (one for which the user has no account), and wishes to post a comment from an account on a home instance (the one on which the user has an account).

It appears the system is currently limited with respect to how the user may find the same post on the home instance.

Each instance appears to assign its own integer identifier to each post, global to the instance but not common with others.

Solution

To begin, it would be helpful if a process were available to construct a web address for the post on the home instance.

For communities, such a process is available.
The same community is represented by https://foreign.tld/c/name and https://home.tld/c/[email protected].

In contrast to the identifiers used for communities, identifiers for posts appear to be integers chosen separately by each instance.

The existing design complicates the means for supporting the requested functionality.

Alternatives

One possibility is to process addresses of the form https://home.tld/post/[email protected].

An implementation might be achieved by the home instance, on the back end, querying the foreign instance to resolve a universal identifier to the post, which would be also available locally, and then issuing a redirection to the address for the same post given by the local identifier.

Another possibility is using a universally unique identifier, such as a hash code, rather than an integer that is unique only locally, in the address normally given for a post.

Notes

The same concerns also may apply to other types of resources, such as comments.

[Nutomic]

You can do this by pasting the post url in the search field. Otherwise see #2987