add_threepid email confirmation emails link to /unstable/ namespace

[richvdh]

When you add an email address to an account, you get an email which invites you to click on a link whose path is https://<server>/_matrix/client/unstable/add_threepid/email/submit_token?token=...&client_secret=....

We should not be using an unstable prefix for stable features. The correct path for this would be something under https://<server>/_synapse/client/.... Of course we'll have to support both for a while to avoid breaking the links in any emails that have already been sent.

(This was added in #6042, and probably should have been cleaned up as part of finalising matrix-org/matrix-spec-proposals#2290)

[DMRobertson]

Patrick notes this and the related /_matrix/client/unstable/add_threepid/msisdn/submit_token on #8334.

I suggest we close this as superseded by that issue.

[clokep]

I think it would be more useful to make the results of #8334 actionable into separate issues.

I suggest we leave this open as a separate task.