Contact Forms: Prefix all fields with a number.

Prefixing all fields by a running number let's us keep the current simple
storing format while supporting having multiple fields of the same name and
type. These prefixes are removed in the admin view, as well as the success
message (though only if the prefix exists to not break previous feedback in
admin).
Contact Forms: Strip label prefix in emails.

Strip the running number prefix from extra fields in emails, but do so only if
such a prefix exists for backwards compatibility.
Contact Forms: Fix tests.

Contact Forms tests now test for the new prefixed format of storings field
names and values
Contact Forms: Fix token subject replacement.

Token replacement from subject now takes into account prefixes in label names.
Contact Forms: Case sensitive prefix stripping.

No need to use a case-insensitive regex for stripping prefixes out of field labels when displaying them.
Merge pull request #652 from rase-/fix/identically-named-fields

Contact Forms: Prefix all fields with a number.