[ActivityPub] Use microformats on mentions

To avoid pointless link previews.
see: https://misskey.xyz/notes/5c51ab5c2d85f2003248eddc

src/mfm/toHtml.ts

@@ -137,6 +137,7 @@ export function toHtml(tokens: MfmForest, mentionedRemoteUsers: INote['mentioned
 				default:
 					const remoteUserInfo = mentionedRemoteUsers.find(remoteUser => remoteUser.username === username && remoteUser.host === host);
 					a.href = remoteUserInfo ? remoteUserInfo.uri : `${config.url}/${acct}`;
+					a.className = 'mention';
 					break;
 			}
 			a.textContent = acct;