When in issue tracker, make right-hand search box use issue tracker filter, and say "Search for issues"

Currently, the website search on the right uses the general Site filter, displaying the text "Search for help," except when in the Handbook, where it uses the Handbook filter and displays the text "Search in handbook."

This request is for the search box to use the issues filter when in the issue tracker—it's currently far from obvious that this filter exists. See https://musescore.org/en/node/99721#comment-459396 for more discussion.