Don't submit prototype fields for repeatables

Related to #624, large forms can lead to data loss depending on the server configuration. When a group or field is repeatable, Fieldmanager outputs a "prototype" for that repeatable element, then new elements clone it. These hidden "prototype" elements are still part of the form and get submitted with the form.

Fieldmanager should either remove them prior to submission via JS, or (preferably) the prototypes should never be a part of the DOM in the first place. A _relatively_ straightforward solution would be to leverage Underscore templates and change the repeatable functions to use the Underscore templates to build new fields.