Port to Backdrop CMS

Thank you for all your hard work on this module

I have ported this to Backdrop CMS.
It can be found at:

• https://github.com/backdrop-contrib/gist_filter
• https://backdropcms.org/project/gist_filter

A corresponding ticket can be found at https://github.com/backdrop-ops/contrib/issues/720

I have incorporated either as is (but ported to Backdrop) or with modifications the following patches:

• regex not working 🐛 regex not working Needs review | patch → by chrisfromredfin | Used as is
• Support authenticated API requests ✨ Support authenticated API requests Needs review | patch → by chrisfromredfin | Updated to use tokens and porting changes.
• Improve Error Reporting ✨ Improve Error Reporting Needs review | patch → by chrisfromredfin | Used as is
• Embedding on HTTPS sites 🐛 Embedding on HTTPS sites RTBC | patch → by arnested | Used as is
• Allow usage of all three display methods at once ✨ Allow usage of all three display methods at once. Closed: duplicate | patch → by MaskyS | This was actually a D8 patch, but was able to upgrade to Backdrop CMS. I also changed it to support using the default method for the text format in addition to the override codes.

In addition, I have modified how the module works with the cache. Gist Filter now has its own cache and only re-fetches from the API if the gist has changed, saving on API calls for big sites with lots of gists.