Teams: Multiple newlines in heading now render as code block

Since a few days ago, the headings of MS Teams adaptive cards sent by Mimir looked strange. What was previously a heading was now rendered as a code block. This change also retroactively changed the look of cards sent before the update.

The issue was that our custom template for the heading added newlines to the beginning of the string. This seems to be rendered as a code block now. Mimir should probably strip whitespace from the heading string to avoid this problem.

Start by locating the custom template that builds the MS Teams adaptive-card heading and reproduce the rendering described in issue #527. Update the heading handling so leading and trailing whitespace no longer turns it into a code block, then verify that headings render normally, including for cards created before the change.

Written by the indexing model from the issue text.