Spike AXE A11Y testing in Cypress (#9111)

* Spike AXE A11Y testing in Cypress

* Fix NewRoomIntro breaking html/aria list rules

* Fix HeaderButtons breaking aria role semantics rules

* missing type

* Switch left panel from aside to nav and include space panel

* Give the page a main heading of the room name when viewing a room

* Use header landmark on RoomHeader

* Improve aria attributes on composer when autocomplete is closed

* Fix aria-owns on RoomHeader

* Give Spinner an aria role

* Give server picker help button an aria label

* Improve auth aria attributes and semantics

* Improve heading semantics in use case selection screen

* Fix autocomplete attribute to be valid

* Fix heading semantics on login page

* Improve Cypress axe testing

* Add axe tests

* Stop synapse after the timeline tests

* Await spinners to fade before percy snapshotting timeline tests

* Improve naming of plugin

* Update snapshots

* Fix accidental heading change

* Fix double synapse stoppage

* Fix Cypress timeline avatar assertions to be DPI agnostic

* Fix aria attributes on date separators

* delint

* Update snapshots

* Revert style change

* Skip redundant call

package.json

@@ -167,10 +167,12 @@
     "@typescript-eslint/parser": "^5.6.0",
     "@wojtekmaj/enzyme-adapter-react-17": "^0.6.1",
     "allchange": "^1.0.6",
+    "axe-core": "^4.4.3",
     "babel-jest": "^26.6.3",
     "blob-polyfill": "^6.0.20211015",
     "chokidar": "^3.5.1",
     "cypress": "^10.3.0",
+    "cypress-axe": "^1.0.0",
     "cypress-real-events": "^1.7.1",
     "enzyme": "^3.11.0",
     "enzyme-to-json": "^3.6.2",