Replace /core/issue with /search #223

- I would like to work on "#128 - Remove /core prefix" after this pull request is merged;
- This pull request also fixes a minor bug at the home page, where the "Search" button didn't respond.
  https://github.com/hyperkang/www.freedomsponsors.org/commit/6a21d8b0863688f71e070412c6643d4b9738ceed#djangoproject-templates-core2-home-html-P13