Fix search

Search bar behaves more like how user would expect. From https://github.com/mapbox/tm2/issues/191, I hit these:

- [x] cmd+f when search is already open should not open browser search.
- [x] Allow user to create a new search based on old search. Right now if a search is in, no way to reset it and start a new search.
- [x] cmd+g only works after search has been started. Cannot start a search with cmd+g.
- [x] Clarify that user must press 'return' to view results. A 'go' button will solve this problem.
- [x] Allow user to press escape to dismiss search UI (currently only works if input is active)
- [x] Do not obscure code with search bar
- [x] When dismissing search, remove all highlights in code.

There are a couple things I didn't do, like dismissing search from tab when switching tabs, and informing user of number of search results or if search result failed.

Would prefer to get this merged them tackle the unfinished points later.

This PR also removes find/replace. It was confusing/unusable in it's prior state.