feat: customize form

[skip ci]
feat($Boilerplate): Add success and error pages

These pages are what the boilerplate generate will redirect you to
on error or succes.

[ci skip]
feat(customize.html): Email field is required
feat($customize): Add more builds

* UI mode
* Paste
* Dnd
* Delete File
* Edit File
* Image Previews

[ci skip]
feat($build/error.html): Show error message based on query string
feat($customize.html): Implement UI tweaks
Merge branch 'site/customize' into feature/4.0-site-update