subscription: Include archived channels in streams list.

`is_archived` field is added to the stream and types.

Include a new `archived_channeels` client capability, to allow clients
to access data on archived channels, without breaking
backwards-compatibility for existing clients that don't know how to
handle these.

Also, included `exclude_archived` parameter to `/get-streams`,
which defaults to `true` as basic clients may not be interested
in archived streams.

zerver/lib/event_schema.py

@@ -50,6 +50,7 @@ from zerver.models import Realm, RealmUserDefault, Stream, UserProfile
 # These fields are used for "stream" events, and are included in the
 # larger "subscription" events that also contain personal settings.
 default_stream_fields = [
+    ("is_archived", bool),
     ("can_remove_subscribers_group", int),
     ("creator_id", OptionalType(int)),
     ("date_created", int),