There's a 'whole collection' link in the top left, so the latter part
is probably not required, and shortening this will bring it into line
with the sidebar placeholder. Open to suggestions on the wording,
just think it's probably best to keep it short.
Also remove unused string.
- Remove _searchPrompt.
- Add placeholder prompt.
- Move search for current card from browser to caller. (Thus, support
current card search even with opened browser.)
