ENH Let AssetAdmin default to Upload_Validator default upload limits

[GuySartorelli]

Description

This change is based on silverstripe/silverstripe-assets#469 (review) and aims to get the other work in a state it can be merged in

Manual testing steps

Set this configuration - it should be respected by asset admin when before it was not.

SilverStripe\Assets\Upload_Validator:
  default_max_file_size:
    '[image]': '500k'
    '*' : '1m'

Then set this configuration - the Upload_Validator configuration should still work in UploadField in forms but not in asset admin - asset admin should now allow up to 2MB uploads for all file types.

SilverStripe\AssetAdmin\Controller\AssetAdmin:
  max_upload_size: 2m

Issues

• Fix clobbering of the upload size validation silverstripe-framework#10058

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