When unwrapping a mention tag into its plain-text form, we are always using the mention tag value:

• <a data-hyp-mention="" data-userid="acct:username@authority">@username</a> -> @username
• <a data-hyp-mention="" data-userid="acct:username@authority">@Display Name</a> -> @[Display Name]

However, this can cause the plain-text version to be outdated if either of those values has changed.

Instead, we should cross-reference the annotation tags with the entries in annotation.mentions, so that we can unwrap to the most recent username or display name, ensuring a valid mention is generated when wrapping them back into a mention tag.