We were using a `height: $line-height` property for this task. One of
the disadvantages of this approach is that things don't look so great
when the label expands over more than one line.
Back when we added that property, browser support for flex layouts
wasn't that great. Now there's universal support for it, so we can use
it instead.
This way we make it easier to modify.
Note that, since the title of the page is "Administration" and it's in
the "Admin" section, we're adding an option to the `header` method in
order to avoid having a confusing title like "Administration - Admin".
Also note that, by removing the `title` HTML class, we avoid inheriting
styles from the `dashboard.scss` stylesheet, and now the heading is
displayed in the position it was meant to.
Finally, the concept of using a `main-class` for the current page comes
from a branch (currently in development) which will replace the <div>
tag with the `admin-content` class with a `main` tag.
