Feedback: add Markdown rendering of feedback messages

[lcjohnso]

Package

lib-classifier

Is your feature request related to a problem? Please describe.

As a project owner / collaborator, I would like to configure feedback success and failure messages (either as single, default messages configured as part of the rule or as subject-specific messages configured in subject metadata via keys like #feedback_N_successMessage or #feedback_N_failureMessage) where the string is rendered using Markdown syntax to allow for embedded images in feedback messages.

Example: when #feedback_N_successMessage is configured as Success! ![mark_check.png](https://panoptes-uploads.zooniverse.org/project_attached_image/c9aec1c1-769f-4a38-a697-57289fa61ba7.png), render the image in the feedback message modal.

To reproduce:

1. Visit https://frontend.preview.zooniverse.org/projects/lcjohnso/lcj-test/classify/workflow/11280
2. Classify on the workflow, where both success and failure messages are configured via subject metadata for "feedback_example_question" SubjectSet (#119218)
3. You will see pop-up feedback message where no image is shown -- raw Markdown syntax is not converted.

Describe the solution you'd like

Render the feedback messages as Markdown allowing for embedded images, links, etc.