LG-746 Return 400 error for invalid String params

[monfresh]

Why:
When a controller uses Strong Parameters such as:
params.require(:user).permit(:email), the user param is assumed to
be a Hash, but it's easy for a pentester (for example) to set the
user param to a String instead, which by default would raise a 500
error because the String class doesn't respond to permit.

How:
To get around that, we monkey patched the Ruby String class to raise an
instance of ActionController::ParameterMissing, which will return a
400 error. 500 errors can potentially page people in the middle of the
night, whereas 400 errors don't.

Note that Devise handles this scenario gracefully in SessionsController,
but because it determines whether or not to call permit based on
whether or not the object responds to permit, the Devise code will
end up calling permit because all Strings respond to it now after our
monkey patch. This is why the invalid_sign_in_params_spec had to
change.

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

• 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.

• 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.

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

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

• 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.

• 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.