Update AssetsEnabledFieldFactory according to UI's RichTextFieldFactory basic image configuration

Basic CKEditor configuration has been brought back in linked issue MGNLUI-2626.

• RichTextFieldDefinition#isImages is now functional also without dam's extended factory
  • CKEditor image plugin is in
  • filebrowser plugin is out
• Magnolia's default RichTextFieldFactory brings a static "complete" Toolbar config
  • We use CKEditor.config.removePlugins to disable features, hide buttons.

The role of AssetsEnabledRTFF is now simply to plug our forked filebrowser plugin and asset-choosing listeners on top of CKEditor image plugin which is covered by default RTFF.