fix(form-builder): move form builder buttons back to bottom of form

[charlieTheBotDev]

Description

An unnecesary prepend used during testing managed to make its way into the last release (making forms work correctly in modals) which meant the form buttons added (submit/additional buttons) were inserted at the start of the form instead of continuing to be placed at the bottom of the form.

As the change was only solving some minor spacing problems, reverting it makes the most sense. I'll follow up with investigation into the modal issue moving forwards.

I also noticed an issue with the modal demo not working as intended due to some refactoring (onClose -> close) so fixed that as well

Motivation and Context

Bugfix

How Has This Been Tested?

Visual testing only in the demo

Screenshots (if appropriate):

Before

After

Checklist:

• [x] I have added a tag to this pull request that indicates the impact of the change (patch, minor or major)
• [x] My code follows the code style of this project.
• [x] I have updated the documentation accordingly. (This includes updating the changelog).
• [x] I have read the CONTRIBUTING document. (Last updated 16 Aug 2019)
• [ ] All my changes are covered by tests.
• [x] This request is ready to review and merge

[coveralls]

Coverage increased (+0.02%) to 69.412% when pulling b7d5aa5ffe387bcde5547c4fc2c94c9e9809e087 on patch into f0f88e6f9583b5830e008de257c4bb2c52efa118 on master.