Per WhisperSystems/TextSecure@8a1428e, bump GIF limit to 5MB, and
audio/video limit to 100MB. Update toast to notify in correct
human-readable units. The only kB size limit is for images, and will
trigger only if after scaling up to 4 times, the rescaled image did not
come in under the size limit without unacceptable quality loss.
Closes#354