Replaces most of the extension's custom css with Bootstrap patterns

Replaces extension's custom CSS with Bootstrap patterns, reducing the custom css in the extension to four selectors - two to reapply bootstrap css that can't be loaded using a boostrap-theme parent ID selector as it would be inherited by the rendered snippet - and snippets need to be able to control whether or not Bootstrap styling is applied or not.

This was discussed in !5 (merged) - and this MR has been rewritten to reflect a simplification in the approach from using a subset of Bootstrap, to putting the #bootstrap-theme selector in several places. It's not good practice to use an ID more than once on a page, but this is the simplest method to both use Bootstrap styling AND not have this rendered by snippets that shouldn't be referencing bootstrap.css. At the future point that Bootstrap in Civi admin isn't namespaced, this can all be removed.

Without a Civi-Bootstrap subset (ie just wrapping everything in #bootstrap-theme)

With a Civi-bootstrap subset

NOTE: because of my messy Git mgmt, this MR also includes adding new snippets.