Fix dots in Wiki slug causing errors

### What does this MR do?

When a user enters in dots into the Wiki slug, an error occurs:

```
NoMethodError (undefined method `escaped_url_path' for nil:NilClass):
  app/models/wiki_page.rb:172:in `set_attributes'
  app/models/wiki_page.rb:191:in `save'
  app/models/wiki_page.rb:155:in `update'
  app/controllers/projects/wikis_controller.rb:49:in `update'
```

This MR fixes this problem.

### Are there points in the code the reviewer needs to double check?

See the problem below.

### Why was this MR needed?

The issue is that the `save` method gets called differently:

```ruby
def create(attr = {})
....
    save :create_page, title, content, format, message
```

or

```ruby
def update(new_content = "", format = :markdown, message = nil)
...
    save :update_page, @page, content, format, message
```

In the create case, title is the slug entered in by the user (e.g. `path/how-to-write-wiki-pages`).

In the update case, originally `@page.page` included the format extension (e.g.`path/how-to-write-wiki-pages.Markdown`). The method `page_title_and_dir` was trying to handle both cases and not doing the right thing. For example, calling `page_title_and_dir('test-1.2.3')` would result in:

```
path_title = test-1.2
path_dir = 3
```

### What are the relevant issue numbers / [Feature requests](http://feedback.gitlab.com/)?

Issues #1263, #431

This replaces !156

See merge request !419