13494: Adds ability to customize the search label on DataExplorer.

13494: Adds ability to customize the search label on DataExplorer.

Also, fixed DataExplorer's default search label as it was showing
'Search files' on wrong contexts (ie: Projects panel).

Also, hides the search feature on the Collection PDH panel, as it doesn't
make sense to search for files on different collections with identical
contents, and also kind of gets stuck when searching for collection names
and having one result, the panel auto navigates to the collection panel.

Arvados-DCO-1.1-Signed-off-by: Lucas Di Pentima <lucas@di-pentima.com.ar>