fix bug 1093122 - Add migrations

Augment LanguageDictValidator so that it is deconstructible and has
sensible equality operations (== / !=).
bug 1098672 - add and populate Feature.mdn_uri
bug 1098672 - Switch feature.mdn_path to .mdn_uri
bug 1098672 - Drop feature.mdn_path from database
bug 1098672 - Update loader for Feature.mdn_uri
bug 1098672 - Update browse app for mdn_uri
fix bug 1098672 - Update documentation