the big message improvements branch

[burnettk]

Summary by CodeRabbit

• New Features

  • Added two new endpoints /message-models and /message-models/{relative_location} to retrieve lists of message models.
  • Introduced MessageEditor component for editing messages within a process group.
  • Added new process models and schemas to demonstrate content display using Markdown and Jinja.

• Bug Fixes

  • Updated ProcessGroupForm to include messages field in the postBody object.

• Enhancements

  • Improved handling of message models and correlation properties.
  • Updated package.json dependencies and references for better compatibility and new features.

• Documentation

  • Added new sections on creating content using Markdown and Jinja in the README file.

• Tests

  • Introduced new test methods for message model list retrieval and process group update synchronization.
  • Added test cases for DataSetupService class focusing on process models and messages.

• Chores

  • Removed outdated methods and definitions, such as update_correlation_cache in spec_file_service.py.
  • Cleaned up BPMN files by removing unnecessary participants and message flows.