Notification is badly displayed in case of Mastodon announcement

Performing a "boost" (= retweet) in Mastodon triggers an Announce activity from it. Right now we are not displaying the received message properly in that specific case.

Reproduction step:

• Follow a mastodon user from XWiki
• Perform a boost on mastodon with this user

Expected result:

• an announce notification is received and properly displayed

Obtained result:

• the notification is received but badly displayed (see screenshot)