Replaces instances of an old hack with the more clear ugettext_noop.

Clicking on conflicting option box unchecks all conflicts
LMS: Clicking license text should bring to new window
updated styles to reflect html reuse inside of xblock edit modal area.

Add ARIA attributes to license for a11y

Gracefully handle re-selecting of selected license

This allows course authors to choose between two difference licenses for their
course content: All Rights Reserved, or Creative Commons. In the backend, XBlocks
that wish to allow custom licenses need only inherit from LicenseMixin, which
adds a `license` field as a string.

License information is displayed in the Studio editor view, and just below the
rendered XBlock in the LMS. In addition, if the course block itself has a custom
license set, this license will display just below the main body of the page
on courseware pages.

This entire feature is gated behind the LICENSING feature flag.