User task improvement and other useful features

• Support for candidate users and groups in UserTask
• Camunda,Flowable: Support for ExternalTask bpmn element
• Flowable: Highlight that ServiceTask is 'triggerable'

What's Changed

• Added PR action by @valb3r in #313
• User task by @v20stepanenko in #312
• Service task trigger by @v20stepanenko in #315
• External task by @v20stepanenko in #314

New Contributors

• @v20stepanenko made their first contribution in #312

Full Changelog: v0.4.9.3...v0.4.9.4