[JB] Temp add Omnibox FF

[thenamankumar]

As per the thread: https://sourcegraph.slack.com/archives/C05AGQYD528/p1738076320715949?thread_ts=1738004127.714799&cid=C05AGQYD528

This PR adds the Omnibox FF back.

THIS MUST BE REVERTED BEFORE THE CODY RELEASE ON 29th.

@umpox

Test plan

• set overrides for ff cody-experimental-one-box on s2
• when the ff is on, the omnibox elements, intent dropdown in the submit button, auto intent detection and search results should work
• when the ff is off, all the omnibox elements should be hidden and auto intent detection also should not work.

Tested manually.