ToC: Use display:table, so that we can behave like a block element

The downside to inline-block is that you don't clear other inline
elements. Although inline elements surrounding the ToC are rare, they
are common enough that this introduced some problems on English
Wikipedia.

Instead, let's just display as table. This is a block element and has
all the boxmodel behavior that we want (that of a table, an intrinsic
wide block element). It's not the nicest perhaps, but it works in all
modern browsers without known problems as far as I'm aware.

We still fallback to inline mode on browsers that don't properly
support display:table.

Followup to: I4ecd0659d1f955a9b593d281a3fef0a81c218a52
Bug: 658
Change-Id: Ib8e2801f9a143de370589728aad01926906ffae0
Merge "ToC: Use display:table, so that we can behave like a block element"