Start annotating which version of the spec added a thing (#3425)

* Introduce a new "added-in" template and use it on endpoints

* Use "added-in" on schema properties too

* Annotate sections of the spec with their added versions

* Demo of "added-in" on a room version (to be fleshed out)

* Use clearer versioning semantics

* Update and fix validator for Swagger custom properties

* Fix docs

.gitignore

@@ -1,9 +1,8 @@
-/api/node_modules
+node_modules
 /assets
 /assets.tar.gz
 /data/msc
 /env*
-/node_modules
 /resources
 /scripts/gen
 /scripts/continuserv/continuserv