This is in preparation for group wikis.

- Extract wiki controller actions into a concern
- Use `container` instead of `@project`.
- Replace `@wiki` / `load_project_wiki` and `@page` / `load_page`
  with `wiki` and `page` getters.
- Add `load_sidebar`, and don't call it in the `pages` action.
- Add `wiki_page_path` routing helper.
- Use `wiki_page` factory in specs and remove manual page setup.