[META] Support for admin forms/pages

Problem/Motivation

We should support admin forms and page a bit more and also offer support for common components like drop down buttons, vertical tabs, etc.

Original discussion here

Proposed resolution

Lets not forget that we only want to minimally fix/support components that we really need to. bs_base is a base theme and not an admin theme.

Remaining tasks

• #2883247: Improve dropdown button/link support
• #2883616: Vertical tabs should use flex layout model so we don't need to use clearfix