load database dump via local file system

As in previous commit, now we copy first the JSON to the chrome file
system before processing it. NB: for large file their is still crashes
probably due to the number of time the JSON is represented in memory in
userStudyToolbox while procesing the file: 1 arrayBuffer to load the
file + 1 int8 array + 1 regular array +1 string. We should avoid crashes
by chunking the reading phase ( loop over the array buffers to read into
the file).
Store the incognito time series

Store as sessions the time when the user went incognito (the extension
one). We also record if the user when idle during this time.

This value is included in the database dump.