Add UI in settings to change ID Server

Just changes the current ID server being used

To come in subsequent PRs:
 * Store this in account data
 * Check for terms or support the proper UI for accepting terms when setting
 * Support disconnecting

Part 1 of https://github.com/vector-im/riot-web/issues/10094
Requires https://github.com/matrix-org/matrix-js-sdk/pull/1013

res/css/views/elements/_Tooltip.scss

@@ -55,7 +55,7 @@ limitations under the License.
     border-radius: 4px;
     box-shadow: 4px 4px 12px 0 $menu-box-shadow-color;
     background-color: $menu-bg-color;
-    z-index: 2000;
+    z-index: 4000; // Higher than dialogs so tooltips can be used in dialogs
     padding: 10px;
     pointer-events: none;
     line-height: 14px;