/_matrix/identity/api/v1/sign-ed25519 links in emails should use non-/_matrix namespace and have a better name

[richvdh]

Currently, email invites contain a link which includes a callback to https://matrix.org/_matrix/identity/api/v1/sign-ed25519 (or the vector.im equivalent). (See template).

This URI appears to be a bastardisation of the now-unspecced /v1/sign-ed25519 endpoint, in which the parameters which should be passed in the body are in fact passed as query-parameters.

Since this is a Sydent URI, generated by Sydent, there is no need for it to be in the spec; however what we should do is move it out of the /_matrix/ namespace (much like Synapse's /_synapse/client endpoints), and document it in Sydent's docs somewhere.

[richvdh]

Also: its purpose is not just to sign some ed25519: it looks up a previously-stored invite. As such the name /sign-ed25519 is doubly crappy.