Auto merge of #1664 - jtgeibel:update/comrak-and-ammonia, r=sgrif

Update to the latest `comrak` and `ammonia` for processing markdown

The latest version of `comrak` requires an explicit opt-in to produce
unsanitized output (the previous default).  We already sanitize the
output with `ammonia` and `comrak`'s sanitized output is too
restrictive, causing 5 tests to fail.

Refs: #1265

Author: bors