Cleanup.
* Fixed preview issue when using an external Collection that does not contain Documents.
* Added `local-sort` attribute on columns definition to operate a JavaScript sort on that column.
* Added button in header of date columns to toggle relative dates display.
Added setting in user profile to define what to do when a Document is saved in an Editor ("back to list" or "stay on form").
Added directive on <time/> elements that allows display of relative dates.
Merge pull request #311 from fredericbonjour/Admin

Improvements in `<rbs-document-list/>`, User profile and dates display.