Trailing slashes required on some federation endpoints

[turt2live]

• PUT /send/$txnId
• GET /pull Unused endpoint
• GET /event/$eventId
• GET /state/$context
• GET /state_ids/$roomId
• GET /backfill/$context
• GET /groups/$groupId/roles
• GET /groups/$groupdId/categories

[neilisfragile]

Proposed implementation is for Synapse to be permissive in what it accepts, ensure that its own requests are spec compliant but if the remote end rejects, then retry on the alternative form.

[anoadragon453]

So what do people think about removing the [^/]* at the end of every servlet's PATH variable and instead consolidating it all into BaseFederationServlet, which'll just stick a [^/]* at the end of the path, where [^/]* means 0 or more /s that doesn't count as part of the pattern.

[anoadragon453]

Sytest is going to need some changes as well in terms of what it expects (/ vs no /).

[richvdh]

reopening this because #4793 got reverted

[anoadragon453]

Done in #4935