Create 1.x-3.x branch from D7 version 7.x-3.5

[bugfolder]

Fixes https://github.com/backdrop-contrib/views_bootstrap/issues/54.

Also addresses https://github.com/backdrop-contrib/views_bootstrap/issues/20.

This is pretty much a complete rewrite of the module, starting from the Drupal 7.x-3.5 version as the base, converting it to Backdrop, then merging in the Backdrop-specific additions that have been made up to the 1.x-1.2.3 version.

I'm submitting the PR against the 1.x-1.x branch so that it can be examined and tested, but for version consistency, I would suggest turning this into a new 1.x-3.x branch, and then this version of the module would be 1.x-3.5, so that its version number matches that of the Drupal version. And then if there are any more Drupal 7 releases, we would follow the Drupal version numbering.

I have tested all of the components under these scenarios:

• Install this version of the module, create a new View with test pages for each of the components.
• Install the 1.2.3 version of the module, create a new View with all supported components, upgrade to this version.
• Install the 7.x-3.5 version of the module in a Drupal site, then upgrade the site to Backdrop using this version.

Of course there are many, many combinations of settings to potentially test and I haven't reached all corners of parameter-space. Further testing is solicited.

A brief summary of changes:

• I used the file organization of 7.x-3.5, rather than the organization of 1.x-1.x, to make it easier to track future changes in 7.x-3.x.
• Implemented all Bootstrap components from 7.x-3.5, including ones not in 1.x-1.x.
• Added hook_update_N() hooks to upgrade from either 7.x-3.5 or 1.x-1.x versions.
• Admin settings form is implemented in the Backdrop standard way, using system_settings_form().
• Added config setting to allow user to choose the version of the Bootstrap library from the CDN.
• Commented out (but didn't remove) the "help" lines in the options settings, which pointed to pages on Drupal.org. Plan is to replace the pages on Drupal.org with pages in the Wiki section of the module's Github page, then uncomment the help information with updated links in a future minor bugfix release.
• Added docblock variables to template files where they were missing or wrong in 7.x-3.5.
• Rewrote and reorganized parts of the README for clarity and/or to reflect some of the changes (like added components).
• Fixed a bug in 7.x-3.5's implementation of the Media object.

Release notes for this version should include a warning that templates have changed since version 1.x-1.2.3, so that if users have overridden any templates they'll need to update them. (See similar warnings in the D7 version release notes.)

[bugfolder]

To assist in testing, I've created a View that contains examples of all of the different Bootstrap Views, making use of the Post content type that comes with a default Backdrop installation.

To test, create several nodes of type Post, including an image (images are used in the Carousel), then install this View, either by dropping it into your active config directory or putting it into staging and then syncing.

views.view.views_bootstrap_test_135.json.txt

You will see all the URLs of the test views on the Views admin page.