Set Font-Family for Dropdown-Back-Link in Top Bar

The **Back-Link** in the collapsed Top Bar is rendered in an `<h5>`. Depending on which **font-family** you are using for your Headers, this could look a little bit weird.
I added an additional variable called `$topbar-dropdown-back-font-family` in which you can set the desired font-family. It defaults to `$header-font-family` which reflects the current behaviour.

**Also:** I fixed the documentation for `$topbar-dropdown-toggle-alpha` which is set to 0.4 in SCSS (not 0.5 as currently documented).