Stop asking whether Element Call is a widget

The remaining reads of the widget global were all asking one of two
different questions, so they get two different answers.

The app shell — the auth hooks, automatic guest registration, the group
call loader's diagnostic and the initial mute state — wants to know
whether Element Call was launched as a widget. That is a property of the
URL it was launched with, so expose the isWidget that computeUrlParams
already computed internally, documented as being for shell use only.

The call interface — whether to offer the profile settings tab — wants to
know something about its host, so it asks the bridge. A host that can
dismiss Element Call owns the user's account, so their profile is not ours
to edit; this reuses the close capability as a proxy, with a TODO
alongside the others.

ClientContext also takes supportsReactions from the bridge rather than
checking four widget capabilities itself, which removes widgetApi from
InitResult — a field that was always null outside widget mode.

Note this changes behaviour for a malformed widget URL: one carrying a
widget ID and parent URL but missing the room, user, device or base URL
would previously have fallen back to registering a guest user, and will
now not.

src/UrlParams.ts

@@ -59,6 +59,17 @@ export interface UrlProperties {
   // Widget api related params
   widgetId: string | null;
   parentUrl: string | null;
+  /**
+   * Whether Element Call was started as a widget of a Matrix client, which is
+   * to say whether it was given a widget ID and a parent to talk to.
+   *
+   * Only meaningful to the standalone and widget builds, which own the URL —
+   * so use it for decisions that belong to the app shell, such as whether
+   * Element Call is responsible for authenticating the user. Anything the call
+   * interface itself needs to know about its host should come from the host
+   * bridge instead.
+   */
+  isWidget: boolean;
   /**
    * Anything about what room we're pointed to should be from useRoomIdentifier which
    * parses the path and resolves alias with respect to the default server name, however
@@ -448,6 +459,7 @@ export const computeUrlParams = (search = "", hash = ""): UrlParams => {
   const properties: UrlProperties = {
     widgetId,
     parentUrl,
+    isWidget,
     // NB. we don't validate roomId here as we do in getRoomIdentifierFromUrl:
     // what would we do if it were invalid? If the widget API says that's what
     // the room ID is, then that's what it is.