ENH leverage FormData to support file upload

[lekoala]

Description

Currently, ajax forms are submitted using serializeArray. This doesn't allow sending more advanced content, like a file upload

Manual testing steps

• Add a FileField in the cms
• Check that $_FILES is empty on save

With this change, files are submitted properly. It also offers the opportunity to do more advanced things (eg: sending binary canvas data as a blob) and, all things considered, should probably work better because it uses a native js feature instead of jquery serializeArray

Issues

#1852

Pull request checklist

• The target branch is correct
  • See picking the right version
• All commits are relevant to the purpose of the PR (e.g. no debug statements, unrelated refactoring, or arbitrary linting)
  • Small amounts of additional linting are usually okay, but if it makes it hard to concentrate on the relevant changes, ask for the unrelated changes to be reverted, and submitted as a separate PR.
• The commit messages follow our commit message guidelines
• The PR follows our contribution guidelines
• Code changes follow our coding conventions
• This change is covered with tests (or tests aren't necessary for this change)
• Any relevant User Help/Developer documentation is updated; for impactful changes, information is added to the changelog for the intended release
• CI is green

[GuySartorelli]

The JS CI is failing as you haven't built the dist files. Can you please build that and include it in the PR?
Instructions are at https://docs.silverstripe.org/en/5/contributing/build_tooling/

[lekoala]

from what i see in the tests, it's not my changes that creates error but i've compiled the files

on windows, it requires this to work
module.exports.rules['linebreak-style'] = ['error', 'windows'];

otherwise you get Expected linebreaks to be 'LF' but found 'CRLF' linebreak-style

could be nice to add that as a default for https://github.com/silverstripe/eslint-config/blob/1/.eslintrc.js to make it windows friendly