Related element-hq/element-meta#1281

Some context in https://matrix.to/#/!oBWlVutBAdQYcYUsAc:sw1v.org/$pnwt-VMwwf3pC7_QET_e2jk_ojfRAPTNZ9fj2ag-Uio?via=jki.re&via=matrix.org&via=vector.modular.im

(This also applies to Sydent, but the work is 50/50 across both projects)

Synapse currently uses the identity server to send 3pid invites out to users (so it sends an email when an email address is invited to a room). However, the templates do not include and do not have a way to include the target web client instance. Instead, all invites include app.element.io.

This is a problem if you are running your own custom Element / Synapse deployment and wish to send invited users to your specific instance, especially so if your instance is not federated.

There are several ways to resolve this:

• Invites could be sent by Synapse (and thus, the template could be configured by the owner).
• Synapse could send the config value of web_client_location to sydent, and the template could use that.
• Synapse could send the homeserver URL to sydent, and sydent could include a parameter to target the right homeserver. This would require Bring back the hs_url parameter and document it element-hq/element-web#5469