We used to render the subscriptions_settings template for every
stream when you loaded "Manage Streams," which can be very slow
for a big realm. Now we only render the right pane on demand.
This makes the subscriptions page responsive by having the settings tab
slide over when a user taps on a stream, giving almost the whole screen
to view the settings.
This is a major change to the /#subscriptions page, converting it to
by a side-by-side list of streams and their settings in an overlay.
There are no new features added/removed, but it's a huge changeset,
because it replaces the old navigation logic and moves the stream
creation modal to appear in the right side of this overlay.
Filter behaves similarly to filter in left sidebar, see PR #684. Added
stream input field to the stream creation modal along with other settings,
for clarity.
Fixes#455, #563.
If a user is not allowed to create new streams, then do not
show the "Create new stream" UI at the top of the settings page.
(imported from commit b97626938d8b612317c2189f7eca0d4bd27fc274)