Remove Entwine-based UploadField (use React-based one instead)

[chillu]

We've created SilverStripe\AssetAdmin\Forms\UploadField with a design overhaul, making the field work in React sections. It currently sits in core alongside the entwine-based 'SilverStripe\Forms\UploadField`. Since we've made the new field useable in non-React sections, I don't see a good reason to keep both fields.

The entwine-based field has broken styling due to some button styling refactor, and uses a legacy HTMLEditor dialog. Since we're moving away from this legacy dialog for the "Insert Media" panel, the entwine-based UploadField would be the last user of this feature - and blocks removing it from core altogether.

• Perform a feature by feature comparison with the old UploadField
• Port over essential features
• Document removed features in changelog
• Set nice error messages on removed features (rather than "method not found")

[tractorcow]

Also the insert-link modal, which allows you to attach a link to a file.

[chillu]

Chris to figure out how we deal with UploadField scaffolding and "insert media" with AssetUploadField being in the asset-admin module (not in framework)

[tractorcow]

See #6667