Right-to-left support

[m-mohr]

Now that we have Arabic in the code base (#571, thanks @randa-11295), we need to work on RTL support.

Todos:

• [ ] Add dir tag to HTML
• [ ] Add config option for direction to config.js (in each locale), with the default ltr
• [ ] Check CSS styles and classes (e.g. ml-... and mr-...) that add padding between elements that they also work in rtl context. Example:
• [ ] ...?