Track additional IdV analytics

[monfresh]

Why: To help us better understand how people use the product and
where they are failing.

The main change here is to capture the controller where the user decided
to cancel verification. This will allow us to see if there's a
particular blocker in the flow. This information is used by other
controllers, so I refactored a bit to reduce duplication.

I also added a visit event for the come back later page, and made sure
that the call to track the visit event in the IdV Sessions Controller
was the first thing in the method. Otherwise, if an error happens before
the analytics call, it wouldn't be easy to tell whether or not the user
visited (or was redirected to) the sessions controller.

For troubleshooting, it's very helpful to have events for all page
visits.

Hi! Before submitting your PR for review, and/or before merging it, please
go through the checklists below. These represent the more critical elements
of our code quality guidelines. The rest of the list can be found in
CONTRIBUTING.md

Controllers

• When adding a new controller that requires the user to be fully
  authenticated, make sure to add before_action :confirm_two_factor_authenticated
  as the first callback.

Database

• Unsafe migrations are implemented over several PRs and over several
  deploys to avoid production errors. The strong_migrations gem
  will warn you about unsafe migrations and has great step-by-step instructions
  for various scenarios.

• Indexes were added if necessary. This article provides a good overview
  of indexes in Rails.

• Verified that the changes don't affect other apps (such as the dashboard)

• When relevant, a rake task is created to populate the necessary DB columns
  in the various environments right before deploying, taking into account the users
  who might not have interacted with this column yet (such as users who have not
  set a password yet)

• Migrations against existing tables have been tested against a copy of the
  production database. See LG-228 Make migrations safer and more resilient #2127 for an example when a migration caused deployment
  issues. In that case, all the migration did was add a new column and an index to
  the Users table, which might seem innocuous.

Encryption

• The changes are compatible with data that was encrypted with the old code.

Routes

• GET requests are not vulnerable to CSRF attacks (i.e. they don't change
  state or result in destructive behavior).

Session

• When adding user data to the session, use the user_session helper
  instead of the session helper so the data does not persist beyond the user's
  session.

Testing

• Tests added for this feature/bug
• Prefer feature/integration specs over controller specs
• When adding code that reads data, write tests for nil values, empty strings,
  and invalid inputs.

[jmhooper]

👏