Enable markdown in all gradethis feedback messages

[gadenbuie]

This PR fixes #152 and closes #176 to add support for markdown in gradethis feedback messages.

There are two places where the feedback message is handled by gradethis. First, the messages in pass(), fail() and related helper functions are processed by glue, either in glue_message() (legacy) or glue_message_with_env() (added in this PR), to interpolate values from the exercise into the message string. These messages are given to the condition object created in graded() that is handled by the grading functions. Eventually, the condition ends up in feedback() where it's used to create the feedback object learnr expects. Here, the condition message is used for the feedback message.

In both places, we differentiate between tag-like, as-is, and other messages:

• tag-like messages are created with htmltools::tags$___() or htmltools::tagList()
  • these messages are returned without modification in the final feedback
  • requires Allow feedback messages to be an htmltools tag or tagList learnr#458
• as-is messages are "protected" by wrapping in I() and
  • are used as-is in the graded() condition
  • are coerced to character and HTML-escaped in the final feedback message
• otherwise messages are
  • collapsed to length-1 strings without whitespace before being processed with glue()
  • are processed as markdown with commonmark::markdown_html() and stripped of potentially harmful tags, e.g. <style>, <script>, or other tags

As a side-effect, message — or incorrect or correct parameters containing a feedback message — can now take vectors which are collapsed without spaces. This could be helpful when writing long feedback messages, but it may have other consequences if the object isn't a character vector.

Edited to reflect final approach.

[schloerke]

LGTM pending small name change