search_box: Improved search box query handler.

[aryanshridhar]

Earlier , The query typed in search box is erased when the user navigates to a new stream . As suggested by @neiljp here , Added a extra keybinding of meta /( alt /) which retains the search query and carries it forward whenever the user changes narrows .

It includes :

• If the user wants to perform a non-sticky search across narrows , the usual keybinding of / would make the job done .
• But , if user prefers for a sticky-search , activating the searchbox with meta / is useful.

Non-Sticky Search : (/)

Sticky Search :(meta /)

Would love an review @neiljp .