XEP-Remarks/XEP-xxxx: Message Reactions

Fallback Body
Summary of the advantages (pros) and disadvantages of adding a fallback body to the reaction.

Pros

 * Users of legacy clients still see the reaction, because to them the message will appear as "standard" message.
 * the legacy body causes the message to become "important" (in the sense of Carbons and MAM) without a hint (this is also a con).

Cons

 * Legacy Client unusable because of reaction spam in MUCs, real messages with content are buried under reactions
 * People might refrain from using reactions, as they notice how they spam others legacy client, which could effectively harm adoption of the feature