Adding timestamp to feature use model

[rboston628]

This PR will add a timestamp to the feature use model.

The goal is to be able to separate out feature usages by time.

Because feature use seems to be an aggregate, I'm unsure of exactly how this will work.

This is a work in progress, and requires more experimentation to ensure correct behavior.

[rboston628]

This needs the migration file. According to @mpatrou this is set up automatically during the build, but I wasn't sure how to initialize it

[sf1919]

@robertapplin and @MialLewis can you help with Staging Server?

[robertapplin]

We might need to add the new column to the admin.py file so we can see it on the admin database page

reports/web/services/admin.py

Line 11 in 2a49519

class UsageAdmin(admin.ModelAdmin):

[robertapplin]

@rboston628 what do we want to use as the default timestamp? When testing the migration locally with

docker-compose exec web python manage.py makemigrations
docker-compose exec web python manage.py migrate

I get the following message:

You are trying to add the field 'timestamp' with 'auto_now_add=True' to featureusage without a default; the database needs something to populate existing rows.

 1) Provide a one-off default now (will be set on all existing rows)
 2) Quit, and let me add a default in models.py