/v2/user/badge/:shortname now also returns evidence field, search improvements

The API will now return the BadgeInstance's evidence URL when requested via /v2/user/badge/:shortname.  This will allow CSOL-site to display the iRemix evidence URL in the badge details page. (addresses https://github.com/mozilla/CSOL-site/issues/760)


Also improved searches on programs and badges (addresses https://github.com/mozilla/CSOL-site/issues/762):

- Searching badges with a search term will now search over badge name, description, issuer name, and program name
- Searching programs with a search time will now search over program name, and all badge names and description of badges within the program.