WIP - PHP 7 upgrade

[niccofyren]

Lots of bigger and smaller tweaks to get rid of PHP 7 warnings and notices. It doesn't take care of each one (more than enough work to go around), but things that seem breaking and encountered while browsing a few of the more common app routes.

Most notable 🧑‍🔧 :

• Got rid of various split references
• Upgraded to newest CakePHP 2.10.x version (which has a bunch of PHP 7 tweaks alongside regular bug and security fixes)
• Changed CakePHP to be installed via composer for smoother upgrade when needed
• Fixed a couple of Dockerfile php extensions incompatibilities with PHP 7

Unrelated 🥕 :

• Adds github action which builds docker image (https://github.com/orgs/gathering/packages/container/wannabe%2Fwannabe-app/223382 only visible to repository users for now)
• Adds editorconfig, so that we can expect at least some basic formatting consistency going forward

I think the only way to properly verify this is to try to run it locally or in a stage environment, doing misc stuff and seeing where any critical errors are thrown. (Misc PHP 7 compatibility tools report no errors, but still a bunch of notices and warnings) To run locally you might have to remove the two git related lines from Dockerfile, to make sure it uses local files instead of master/another branch.