Split translatable text blocks and mark new resources

 * Split long blocktrans strings and exclude unnecessary markup
 * Preserve existing translations for split strings
 * Mark new strings for translation

**Important**

This update modifies a few .po source files. The English master should be pushed to Transifex.
In order to save modified text blocks, these languages should also be updated on Transifex: `es`, `es_MX`, `fr`, `it`, `pl`, `pt_BR`, `ru`.
Finally, pulling the updated source files from Transifex should sync all other languages. I don't know how long it takes for Transifex to update their indexes, so I can submit a new PR for this, if you prefer.

Resolves #241