Stop injecting field errors where they won't be used.

What does this MR do?

Filters the form elements which gl_field_errors validates, excluding input types button, submit and checkbox.

Why was this MR needed?

This won't make a difference in UX, but I just noticed that we are currently validating and injecting errors into the DOM for all non-hidden inputs. Doing so is unnecessary, and could introduce performance problems on forms with larger numbers of inputs.

Does this MR meet the acceptance criteria?

• All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together