search_streams: Add | as OR-operator.

[cathmer]

As requested in #11797 I also added | as an OR-operator when searching streams.

@timabbott When I looked for all occurrences of split(",") I actually found two more places where this is used. One is when creating a new stream and searching for users to add and the other one is used in subs.js on line 384 (to be exact, this is where a method is called from search_util.js which splits the search term using a ,), but it's not exactly clear to me where this is used. Do you want these two occurrences to also use both | and , as an OR-operator? And if so, should I just add that to this commit or make 2 separate commits and PRs for it?

[zulipbot]

Heads up @cathmer, we just merged some commits that conflict with the changes your made in this pull request! You can review this repository's recent commits to see where the conflicts occur. Please rebase your feature branch against the upstream/master branch and resolve your pull request's merge conflicts accordingly.