Add support for keywords in searchbar

[mounilKshah]

What does this PR do? This PR is to add support for keywords in search bar.

Fixes #638

Channel discussion: Expand search support #T638 #T1258

Tested?

• [x] Manually
• [x] Existing tests (adapted, if necessary)
• [x] New tests added (for any new behavior)
• [x] Passed linting & tests (each commit)

Commit flow

• One commit: Support only for stream and topic keywords

Notes & Questions

• This is a WIP PR. It currently only has support for stream and topic name keywords.
• As per the discussions, the plan ahead would be to:
  • Extend support to other keywords for narrows supported by ZT
  • Provide auto-populated text for the current narrow in the search bar
  • Write tests