This review allows users to update attachments with new version. See https://reviewboard.hackpad.com/Reviewing-attachments-tbfh4oiwTHB for description or https://www.youtube.com/watch?v=anX_rQKWSp0 for a quick demo.
Review includes:
* Change to a schema to track sequences of file attachments
* Modified attachment creation code to correctly assign the new parameters
* Modified display logic to only show the latest element of each attachment sequence
* Modified attachment review pages to include the slider
This review is aimed at project branch (see testing section for the problems description)

Performed various scenarios of adding/updating/reviewing attachments.
Current problems:
* There is no beautiful UI
* We are reloading the page each time slider is changed - API should be extended to load the necessary data dynamically.