Add mfa_enabled field to Users table

[monfresh]

Why: To make it easier to query how many users have configured MFA.

For example: User.count(:mfa_enabled)

Note that you want to use the above command, not any of these:

User.count(&:mfa_enabled)
User.count(&:mfa_enabled?)

The former does SELECT COUNT("users"."mfa_enabled") FROM "users"
whereas the latter ones use Ruby's Enumerable, which is a lot slower.

I believe the former is also faster than this:

User.where(mfa_enabled: true).count

What I don't know is whether the former benefits from an index.

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.

[monfresh]

This is a work in progress to get things started for LG-802. I probably won't have time to add tests before I leave.

[monfresh]

Things to consider:

• Does SELECT COUNT("users"."mfa_enabled") FROM "users" benefit from the index on the mfa_enabled column? If not, we should remove the migration, which should save us headaches in production.
• If the index is needed, then we'll need to remember to update the statement timeout env var in production to something higher than the last migration that caused downtime (>2 minutes I think).
• Does the new column have to be deployed separately from the code that writes to it?

[monfresh]

We also need a script to backfill the mfa_enabled column for all users who have MFA configured.

[jmhooper]

Closing this for now. We are looking at solutions for this that don't involve adding more indexes to the user table.