Add support for defining clear button text.

A new translations to be added:

- UserDefinedForm.CLEARBUTTON
- UserDefinedForm.TEXTONCLEAR
Merge pull request #171 from Focus-Flow/feature/clear-button-text

Add support for defining clear button text.
BUG Performance improvements of SubmittedFormField queries.

When there are a lot of SubmittedForm records the UserDefinedForm
page takes a long time to load in the CMS, and oftens exceeds
the PHP memory limit well beyond 128M.

Previously UserDefinedForm::getCMSFields() would build a list of
name => value from all SubmittedFormField records, but it would
do this twice, once in getCMSFields() and another time in
UserFormsGridFieldFilterHeader. It would also use the full ORM
to build this list, when all it needs is a map of the Name
and Value columns.

This fixes that to build the columns once in getCMSFields() using
DB::query() and it'll pass those columns along to
UserFormsGridFieldFilterHeader as well so it doesn't do it twice.
Merge pull request #172 from halkyon/performance_improvements

BUG Performance improvements of SubmittedFormField queries.
Include dev-* support for userforms
Add translations for clear button.
Make sure tests pass after adding clear button.
Merge pull request #173 from Focus-Flow/bug/clear-button-translations

Add translations for clear button.
Merge pull request #174 from Focus-Flow/bug/clear-button-test

Make sure tests pass after adding clear button.