Add a page title to every page.

![You get a title, and you get a title; everyone gets a title!](https://i.imgflip.com/kvmq8.jpg)

The `page_title` helper pushes the provided string at the end of the title, but because of the order that layouts are rendered in by ActionView, the result is always this:

```
<title from view> |
<title from as specified in the controller or by its layout> |
<title from layouts/_head>
```

For example: `Merge Requests | GitLab.org / Gitlab Community Edition | GitLab`.

All a developer needs to know is to put a `page_title` call describing the page in question at the start of every template.

To get everything where I wanted it to go without too much duplication, I had to make some changes around layouts, sidebars and controllers.

See merge request !593