Currently AngularGettextHTMLParser parses translations of form
{$ 'content' | translate $}. However, there is another form in
projects' html files: {$ ::'content' | translate $}.
This commit fine-tunes the filter_regex and related handle_xxx
functions to extract both forms of translations.
Change-Id: I7129a13d046b699328b77267ae4936d31af8144c