This extracts the feature specs for project wikis into shared examples,
and reuses them for group wikis.

Most of the specs were ported directly with some small adjustments to
make the wiki container generic:

- Using the `wiki_path` / `wiki_page_path` route helpers.
- Using `wiki.container` instead of `project`.

Some specs needed special treatment:

- `user_views_wiki_empty_spec` tests the different conditions for the
  empty state message, which on projects can also link to the issues
  tracker. This was refactored into a shared example that can test
  the various conditions dynamically.

- `user_creates_wiki_page_spec` and `user_updates_wiki_page_spec` were
  repeating some tests for project wikis in group and user namespaces.
  This duplication was removed, since these go through the same code
  paths and don't apply to group wikis.

- `user_creates_wiki_page_spec` also contained tests for the wiki
  sidebar, these were extracted into their own shared examples.

- Diffing wiki pages doesn't work yet with group wikis, so those tests
  are temporarily skipped.