Change to use injector for file upload fields in framework

[flamerohr]

Created a FieldHandleField interface to use in place of UploadField (which had been pushed to a dependency), and use FileField by default in framework.

Requires:

• BUG Prevent top-level files being created with incorrect parent folder silverstripe-behat-extension#145
• API Remove tests for asset-admin only behaviour silverstripe-cms#1740 (depends on behat pull request)
• Use injector to place UploadField silverstripe-asset-admin#387 (but merge this AFTER the above two, and THIS framework PR)

[dhensby]

The CMS behat tests are failing - I guess this is related?

[tractorcow]

CMS tests need to be fixed separately. I'll come up with a patch.

What we want is an annotation on cms behat fixtures that disables certain tests when run with asset-admin installed, and then focus those tests (e.g. on upload field) for running in isolation.

[tractorcow]

E.g.

@assets
@excludeIfModule asset-admin
Feature: Insert an image into a page
  As a cms author
  I want to insert an image into a page
  So that I can insert them into my content efficiently

[tractorcow]

If @sminnee can approve this, I've opted to remove this feature from the default insert link dialog. The issue is that react components don't work within any jquery-ui dialog due to the way that events are blocked by jquery UI (in a way that cannot be fixed without modifying jquery-ui directly).

The affected CMS tests are removed with silverstripe/silverstripe-cms#1740

[tractorcow]

File media dialog is now removed; asset-admin module adds it back in. See silverstripe/silverstripe-asset-admin#387

Note that the CMS test removes the insert-media test fixtures, as these are now both implemented and tested via asset-admin.

[tractorcow]

Added fix for behat silverstripe/silverstripe-behat-extension#145

[flamerohr]

This property is unused and inaccessible, we could remove this

[flamerohr]

This is also unused and inaccessible, could remove it

[flamerohr]

we could move this to the asset-admin repo instead right?

[tractorcow]

Feedback addressed; moved the plugin to asset-admin module.

[flamerohr]

@tractorcow 's changes to my pull request look good, I think this is ready for merging