**What does this do?**
It adds validation to the creation of a wiki page, that way the user gets real
feedback instead of just a 404 page if the name of the wiki page was invalid

**Why is this needed?**
There are a lot of characters that are not allowed in the creation of a wiki
page, there is even a small text that is saying: Please don't use spaces.
Although we have that text there, we don't actually validate on this. This
commit adds validation on the title and gives the user actual feedback.

**What issues does this fix?**
Fixes http://github.com/gitlabhq/gitlabhq/issues/5357
Fixes https://github.com/gitlabhq/gitlabhq/issues/8565
Fixes https://github.com/gitlabhq/gitlabhq/issues/3913
Fixes https://github.com/gitlabhq/gitlabhq/issues/8166Signed-off-by: Jeroen van Baarsen <jeroenvanbaarsen@gmail.com>