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

src/components/structures/auth/Login.tsx

@@ -600,10 +600,10 @@ export default class LoginComponent extends React.PureComponent<IProps, IState>
             <AuthPage>
                 <AuthHeader disableLanguageSelector={this.props.isSyncing || this.state.busyLoggingIn} />
                 <AuthBody>
-                    <h2>
+                    <h1>
                         { _t('Sign in') }
                         { loader }
-                    </h2>
+                    </h1>
                     { errorTextSection }
                     { serverDeadSection }
                     <ServerPicker