Translation mechanism for elements and element admin forms

Element metadata is now organized by language, and all elements can/should be
presented language-neutral format. Digital Purchase and Email Collection elements
have already been converted to the new format.

All metadata fields are available to the admin template prefixed by "element_"
so like {{element_label_name}} — all other (non-admin) templates have metadata
available prefixed by "metadata_" so {{metadata_copy_whatever}}

Those are all available automatically.
Merge pull request #297 from jessevondoom/master

Translations for elements