Start role year selects at this year, not 1700

* Starting from 1700 and having to scroll all the way down to 2014, or
even 2019, is painful
* Restrict date entries to current year rather than current year + 5
which seemed to be the default
Refine role copy

* The second part of the sentence was hard to understand
Merge pull request #1658 from alphagov/improve-role-date-selection

Improve role date selection