[bootstrap][policy] Figure out versioning

Problem/Motivation

We released 7.x-3.0 back in Nov 2013. A [production] release hasn't been made since. Ultimately because of this lack of quick releases, this has added to the pain of versioning. There has been many bug fixes, feature additions and even some backward compatibility breaks (#2128129: Provide setting to change sidebar/content widths and https://www.drupal.org/node/2229627). Drupal version standards dictate that we should increase this to 7.x-4.x.

We have, however, in the past made a clear distinction/correlation between the major Drupal version and the external Bootstrap framework major (ie: 7.x-2.x === BS2 and 7.x-3.x === BS3). With the future Bootstrap 4 coming (likely at the end of the year), this could cause confusion as to which "Drupal version" you should install if we break this correlation and go with 7.x-4.x instead of 7.x-3.1.

It does not help that Drupal restricts 7.x projects to only a MAJOR.MINOR version. Semantic versions are/will be only available for 8.x.

Proposed resolution

1) We stick with Drupal versions and explicitly define which branch(es) each BS version correlates to on the project page.

- or -

2) We stick with the correlation between the Drupal and Bootstrap majors (for easier implementation and code bases) and we rollback features (#2128129: Provide setting to change sidebar/content widths) that caused backward compatibility breaks to that a minor doesn't break a sub-theme.