Various Discord References not Converted

[jabernat]

Message contents include the following Discord-specific references that won't make sense to email recipients:

• Custom emojis like <:murdock:1194974822049001543> could be replaced with image tags.
• Channel references like <#1408259522664992778> could be replaced with their name like #Example.
• Role references like <@&490565550876131328> and user references like <@193247376931028993> could be replaced with names like @Captains or @Joe Smith.