This completes the remaining work required to support
addition of all members of another stream.
This allows the creation of stream pills on pasting
the #streamname and copying it from the stream pill.
The user pills uses email ids instead.
And also allows creating stream pills when the user
hides the typeahead.
Tested by commenting out the "set_up_typeahead_on_pills"
line in `stream_edit.js`.
A `node_tests/stream_pill.js` file has been created
for the node tests and the other half of the coverage
check takes place in `node_tests/stream_edit.js`.
We update the pills typeahead logic to also include
stream results and pass the "stream" key in `opts`
to enable this option for the Add subscriber form.
This commit implements the feature of adding all the
subscribers of another stream in the "Add subscribers"
UI, with the help of a new "stream_pill.js` file.
We temporarily add `user_pill.js` to the EXEMPT_FILES
list as typeahead will be set up in `stream_edit.js`
file which does not have any dedicated tests file.
Work towards #15186.
Ryan Rehman