pullrequests: prevent XSS in 'Potential Reviewers' list when first and last names cannot be trusted

If a user first or last name contains javascript, these fields need proper
escaping to avoid XSS attacks.

An example scenario is:
- the malicious user creates a repository. This will cause this user to be
listed automatically under 'Potential Reviewers' in pull requests.
- another user creates a pull request on that repository and selects the
suggested reviewer from the 'Potential Reviewers' list.

Reported by Bob Hogg <wombat@rwhogg.site> (thanks!).


Technical note: the other caller of addReviewMember in base.js itself does
_not_ need to be adapted to escape the input values, because the input
values (oData) are _already_ escaped (by the YUI framework).