Allow renaming messages (from "new message") in message/ broadcasting blocks

[thisandagain]

Reported via internal playtest 3/3/17.

Expected Behavior

You should be able to rename the message to a custom name, and that should show up in the broadcasting blocks' menus.

Actual Behavior

In broadcasting blocks, "new message" is just "new message" (rather than letting you set the name)

Operating System and Browser

Mac & Chrome

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Moved from scratchfoundation/scratch-gui#189
/cc @jwzimmer

[rachel-fenichel]

Depends on #825