[#1039] Adding a reference section for the global twig variables

Hi guys!

See the discussion on #1039. This adds a reference section for the variables that are available everywhere (but doesn't cover field-specific variables).

We already have a section above this that explains "form variables", and we have a link from the "Form Customizations" cookbook entry to this. If anyone sees any other good places to add a link to this or any ways we can make this more clear, please let me know!

After merging into 2.1, I'll update it with the 2.1 changes.

Thanks!