Show search-key & prefix in search boxes if search is enabled

[neiljp]

We conveniently show the keys to jump to search boxes, and when typing text it should be clear that this is filtering the results, or maybe just after pressing 'enter', also hopefully obviously.

However, once the search is enabled, ie. the list of items are filtered, we should probably show the key at the end of the box again, though the actual search text of course will not include this key text. This makes it clear to users that they can go back to the search.

It would also be good to show that a search is 'in progress'/'enabled', by prefixing some text. This is similar to the approach taken in the message list when searching/filtering messages.

[kaustubh-nair]

@zulipbot claim

[kaustubh-nair]

@zulipbot abandon Will take it up after finishing current prs