WL-2247 Wiki servlet handles login requests

When processing a get request the wiki servlet never considered the
possibility that someone might have been trying to login, besides the fact
that it would quite happily send someone on their merry way to the login
helper.

We now perform this check and handle the authentication approrpiately. The
code was judiciousy borrowed from the `AccessServlet`.
Merge pull request #1 from ox-it/WL-2247

WL-2247 Wiki servlet handles login requests