rhbz960786 Add REST resource which exports translation memory for a project iteration
rhbz960786 Use LocaleId for SourceContents.getTargetContents()

With the current Hibernate mappings, exporting a single locale is just as
expensive as exporting all locales, but we don't expect to be exporting the
entire database for just one language very often, and it simplifies the code
considerably.
rhbz960786 Use same strategy whether exporting one or all locales to TMX