Add incoming message corrections

[raucao]

Works in combination with sockethub/sockethub#649 (and will simply show additional messages when used with Sockethub versions that do not support it).

Adds the stanza ID as a new, separate property for message objects (needed for MAM), while the Sockethub change switches the ActivityStream object's "id" property to the XMPP <message> element's "id" attribute.

Left to do:

• Only allow correcting the last message
• Correction must come from the sender of the original message
• Indicate in the UI that a message has been edited/replaced

refs #245

[galfert]

Changes look good and confirmed it to be working 👍