Changelog for March 29

Spring break was last week, but that didn't stop us from getting some things done.

Big stuff:
* Email notifications: You can now choose to have Pipeline send you email notifications of your choice. To edit your notification preferences, go to your profile page and look for the Preferences panel. Current notification options include:
- notify when your application to join a collab is reviewed
- notify when you are invited to join a collab
- notify when someone comments on your submission
More notification types will be added over time.
* Buttons: when you click a button, it will only submit the first time, no matter how many times you click the button. This should eliminate a number of issues we saw with people accidentally posting a comment multiple times, uploading the same file multiple times, etc.

Small stuff:
* File uploads and new comments are now added via a popup dialog box, consistent with adding new scenes and submissions and uploading profile pictures.
* Fixed a bug where users could delete submissions from the Overview tab. This is disabled now; the Overview is geared towards viewing, not editing.

In the works:
* We're working on an overhaul of comments and discussions. The new system will make it easy to have conversations, i.e., reply to specific people and stay on topic.