dev/core#522 CRM-19767 dev/core#204 Add Case Tokens to emails

[seamuslee001]

Overview

This adds in the case tokens to emails as per PR #11291 also incorporates the unit tests for that PR from @varshith in ticket https://lab.civicrm.org/dev/core/issues/204

Before

No case tokens can be used in PDF letters or once off emails

After

This adds the ability to use case tokens on PDF Letters and single emails

ping @colemanw @jamienovick @eileenmcnaughton

[civibot]

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[mattwire]

@seamuslee001 Is it possible to use the new token processor here? See eg. #14662

[eileenmcnaughton]

@seamuslee001 I'm OK with this as a reviewers cut if you are familiar with what this does & confident in it. Agree it's good to do what @mattwire says but ok to leave out of scope given this adds the test

[seamuslee001]

Merging based on @eileenmcnaughton 's comment and @mattwire i think there is going to be lot of refactor to these functions to make that happen