Admin dashboard: Show WHEN people use the tool

[reginafcompton]

Include a chart that shows WHEN people use the tool.

[reginafcompton]

We can accomplish this by using a timeseries from Highcharts and doing something along these lines:

import pandas as pd
from collections import Counter
from django.core.serializers.json import DjangoJSONEncoder

# Get all the sessions (eventually: differentiate between web and text messaging tool), and
# put last_modified date in a list (consider rounding to nearest half hour)      
sessions = Session.objects.all()
dates = [session.last_modified for session in sessions]

# Use pandas to create a time range for every 30 minutes (?)
index = pd.date_range('2018-11-30', '2018-12-1', freq='30min')

# Put the date range in a pandas data structure, and make counter   
df = pd.DataFrame([0] * len(index), index=index)
df.update(pd.DataFrame.from_dict(Counter(pd.to_datetime(dates)), 'index'))

json.dumps(df, cls=DjangoJSONEncoder)

Inspiration from: https://stackoverflow.com/questions/43416099/time-series-from-list-of-dates-python