It seems most of these blacklisted tests do actually pass most of the time.

I'm of the opinion that having them blacklisted here means there is very little incentive for us to deflake any flaky tests, and meanwhile any value in those tests is completely lost.

Dendrite's implementing MSC2753 over at https://github.com/matrix-org/dendrite/pull/1370 to prove the implementation for MSC purposes, and so sytest has sprouted tests for it over at https://github.com/matrix-org/sytest/pull/944. But we don't want them to run on synapse until synapse implements it.

Dendrite's implementing MSC2753 over at https://github.com/matrix-org/dendrite/pull/1370 to prove the implementation for MSC purposes, and so sytest has sprouted tests for it over at https://github.com/matrix-org/sytest/pull/944. But we don't want them to run on synapse until synapse implements it.

Back out cross-signing code added in Synapse 1.5.0, which caused a performance regression.

Implements MSC2290. This PR adds two new endpoints, /unstable/account/3pid/add and /unstable/account/3pid/bind. Depending on the progress of that MSC the unstable prefix may go away.

This PR also removes the blacklist on some 3PID tests which occurs in #6042, as the corresponding Sytest PR changes them to use the new endpoints.

Finally, it also modifies the account deactivation code such that it doesn't just try to deactivate 3PIDs that were bound to the user's account, but any 3PIDs that were bound through the homeserver on that user's account.

As per [MSC2290](https://github.com/matrix-org/matrix-doc/pull/2290/files#diff-05cde9463e9209b701312b3baf2fb2ebR151), we're dropping the bind parameter from `/account/3pid`. This endpoint can now only be used for adding threepid's to the user's account on the homeserver.

Fixes https://github.com/matrix-org/synapse/issues/2306

* Add a sytest blacklist file

* Add changelog

* Add blacklist to manifest