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?

- [x] All builds are passing
- [x] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if it does - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !6929