Improve UI of external storage configuration

 * Highlight fields that need to be filled
 * Remove (required) and (optional) strings where possible
 * Focus first configuration field when creating new external storage
 * Add ability to force external storage re-check
 * Various jQuery improvements
 * Prevent wrapping of checkboxes in configuration
Merge pull request #10428 from owncloud/files_external_better_config

Improve external storage configuration UI