Enhanced RC: Add a "view changes since my last visit" link

It's only shown if there are any unviewed changes and if as least one
of previous versions was viewed already.

* Added new message 'enhancedrc-since-last-visit' for link text
* Added 'enhancedrc-history' to replace 'hist' in enhanced RC context

Bug: 51901
Change-Id: Id74fe88bb2a1879651935ac56e9cdcb2e54b331f