Add string interpolation to the translator.

* The translator looks for strings with %s and %(named)s and replaces with values from context.
Combine disparate strings for easier l10n.

* Interpolate on %s and %(name)s that the Mozilla l10n team is accustomed to using.
* Write email HTML from JS to simplify the translator's job

fixes #267
Merge pull request #333 from mozilla/issue-267-combine-strings-l10n

Issue 267 combine strings l10n