Initial licenses work, Fixes #1469

This pull request fixes upstream issue #1469.

The included license enhancement is the first part of a broader feature set that will include enhancements to licenses and distribution restrictions.  This pull request makes minimal changes to functionality and does not significantly clutter existing UI interfaces.

This pull request adds useful default licenses to GeoNode, including Public Domain, ODbL, and NextView.  The included licenses are a preliminary list and there is no specific need to keep them all in the core GeoNode repo.  We may want to simplify the list or included license descriptions.

This pull requests provides users the ability to expand a license on a document, layer, or map detail page (by clicking a trailing question mark) to see the description of the license and link to its specified url.  This feature is important for users that may be unfamiliar with a license, such as ODbL or NextView.  Admins  can edit the setting file to show the license UI "above" the fold, "below" the fold, or not show it at all.  Admins can also decide to include the license description in the metadata XML or just the existing functionality of name and url.

This pull request also includes code to search over licenses.  However, there are some errors, so it is disabled and hidden by default.  Integrating license search into haystack successfully is beyond what I can accomplish on my own.

There may be edge cases with the PYCSW implementation that I may not be aware of.

I also am unclear about how to do migrations.  Any help would be great.