Address regressions in Email-ext v2.29 due to new dependency on token-macro-plugin: backslash-escaped terminators in content token string arguments no longer work; and very long content string parameters trigger StackOverflowErrors (JENKINS-14132)

- Allow backslash-escaped terminators in content token string arguments
- JENKINS-14132: Very long content string parameter triggers a StackOverflowError

Tests included for both of the above that fail without the changes and succeed with them.