Add support for copying code snippets

Often it's useful to extract code - or maybe any quoted text? - from a message.

The likely initial way to implement this would be to ultimately add an extra (optional) section to the message information popup. However, as with in-message links or topic links, this feature requires solid implementation of extraction of code blocks first into a format the popup can show.

Things to consider:

• each code section may have a language (eg. bash, python), which it would be good to indicate in an entry
• we don't want to list the full section in the message information popup, just enough to be unique
• in the short term, activating the entry for the code could copy it
• in the longer term we may want to extend the copy feature to other links too, and activation could instead trigger a feature like #985

NOTE: I've marked this as a good first issue, but this may require a deeper dive into multiple areas!