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.
  Here is the modified line:
 https://github.com/hyperkang/www.freedomsponsors.org/blob/6a21d8b0863688f71e070412c6643d4b9738ceed/djangoproject/templates/core2/home.html#L67