use slug as wiki title

If you create a wiki page, its slug will be parsed by an unhyphenize method and the first letter will be capitalized. The result is used as its page title which is (in most cases) really ugly.

Example:

SLUG: gitlab-ci-cd-integration
Title: Gitlab ci cd integration
Fancy: GitLab CI CD Integration

I would recommend to simply use the full slug itself as a title. If somebody wants a fancy title for a wiki page, he can simply use H1:

# Fancy Title
...

To-Do

Only app/models/wiki_page.rb needs to be edited by setting title = slug.