Bug 929027 - [Messages] Multiple image sharing from Gallery to Messages r=schung

Updated review comments
Follow-up:

* do not show max size alert for images
* changed slightly the tests
Follow-ups:

* change the string when there is several files
* remove useless Mock_alert
* indentation changes
Merge pull request #17658 from julienw/929027-multiple-share

Bug 929027 - multiple share, r=schung