Simplify and rename helper method

The new name better reflect what the method is returning. The method was
also over-complicated:

* It had a boolean flag switching whether it returned a link or just
plain text, but there is nowhere in the app that uses it to generate a
link.
* No `current?` role appointment will ever have an ended_at timestamp,
so the ternary operator check is redundant.
* The helper is only used in the admin interface, so localising the
date is not required.
* There is no HTML in the text, so marking it html_safe is unnecessary.