Separate get_current_version() into functions to retrieve actual version and one to retrieve its wp-parser-since term object

* get_current_version() now returns a string of the actual version of parsed WP code (or best effort)
* Prefer ‘wp_parser_imported_wp_version’ option value as set in newer version of parser
* Add get_current_version_term() to return wp-parser-since term object for the major version of parsed WP code
* Add argument to permit get_current_version_term() to return term object for actual version
Add link to core.trac to view code source for item. props DrewAPicture

* Change “Source:” label to “Source file:”
* Add “View source...” link that points to relevant line in core.trac when appropriate
In get_current_version(), use the global wp_version instead of a hardcoded version number, unless wp_version isn't a release version. props keesiemeijer