Date video

Timestamps as returned by the API should be in a JS friendly format, not as strings.

* Front end code from Christian
* Changing Attachment table's 'date' column to a Datetime, rather than a String
* Requires a (destructive) migration
* other than the type change, backend as submitted by Christian 


[CYB-220 Videos should show the date posted](https://jira.ets.berkeley.edu/jira/browse/CYB-220)