Add angular-translate for i18n of strings

Summary:
* add angular-translate
  * see: http://angular-translate.github.io
  * whenever possible, we are using mainly this style:
    * <ANY translate>TRANSLATION_ID</ANY>
* INFO: From now on, please avoid direct usage of inline strings when they could be moved to the translation file(s)
* add locale-config.js and call it in app.js for configuring the languages
* add locales folder for translation files
  * use en translations file from backbone app for initial data
  * add some more keys/translations to the en translation file
* replace all inline texts in html teplates with translations directives / filters

Test Plan:
* run 'npm install' (for installing angular-translate)
* check if site looks still the same
* try to modify/add the translations by yourself

Reviewers: seth, rjmackay, shadowhand

Reviewed By: shadowhand

Subscribers: aMoniker

Maniphest Tasks: T967

Differential Revision: https://phabricator.ushahidi.com/D445