New header dropdown / notification menu styles

[Moin]

@jordan-vidrine

Here it is with nav on top

This looks very similar to the menu we had before the last update.

@Jagster

we will move from this

to that:

@keegan

If we’re going with nav on top, could we try it styled like our existing designs we use in horizontal-overflow-nav elsewhere in core.

Don’t these already cause confusion for users on mobile? (For example, when people fail to find the admin button on user profiles). If the last icon was hidden like that, it will be very difficult to find, for example, the logout button.