[Sharing dialog] Allow multiple email addresses to be pasted in at once

[gino-m]

@jabramowitz5 pointed out in today's PM/UX meeting that the UX designs seem to imply this is the desired behavior:

@lecrabe @kenstershiro FYI

[gino-m]

@lecrabe @kenstershiro P2 TODO for GA?

[kenstershiro]

Major timesaver for each new survey, and for training