Do not apply mention detection inside code blocks

Given annotation markdown containing mention-like text inside a code block:

Hypothesis will try to parse `@foo` as a mention, converting into a tag:

We have formulated the pattern used to detect mentions to try and avoid this happening frequently when "@" appears in the middle of text, but there are still cases like the above where it can happen.

Aside from code blocks, we may also want to exclude block quotes and math.