We are using `can_add_subscribers_group` instead of
`invite_to_stream_policy` to check whether a user can subscribe other
users.
We've removed `invite_to_stream_policy` from the frontend wherever
applicable.
This commit replaces the spectrum color picker used in the stream
popover and stream settings with a custom color picker popover, which
contains a grid of predefined color swatches and a custom color option.
The custom color option uses the HTML5 <input type="color"> which shows
the native browser color picker UI.
Fixes#14961.
