Do not report exceptions rescued by rails

[AlanGabbianelli]

TL;DR: This stops exceptions rescued by rails from appearing in Sentry.

After upgrading frontend to rails 6, we started seeing this error:
Mime::Type::InvalidMimeType "charset=utf-8" is not a valid MIME type
Example here:
https://www.sentry.io/organizations/govuk/issues/1549200925/?end=2020-03-06T12%3A00%3A23&environment=staging&project=202225&query=is%3Aunresolved+url%3Ahttps%3A%2F%2Fwww-origin.staging.govuk.digital%2Freport-an-unregistered-trader-or-business&start=2020-03-05T16%3A07%3A23&utc=true

This is because, even though these errors are rescued by ActionDispatch,
they still reach Sentry. Here's the explanation from Sentry's docs:

Rails catches exceptions in the ActionDispatch::ShowExceptions or
ActionDispatch::DebugExceptions middlewares, depending on the
environment. When rails_report_rescued_exceptions is true (it is by
default), Raven will report exceptions even when they are rescued by
these middlewares.

So, in order to stop this behaviour, we initially took a double
approach:

• set that option to be false for frontend in
  this PR to stop all
  exceptions rescued by rails (not only Mime::Type::InvalidMimeType)
  from appearing in Sentry for frontend
• explicitly added Mime::Type::InvalidMimeType to the list of
  excluded exceptions in
  this PR to stop
  Mime::Type::InvalidMimeType from appearing in Sentry for all apps that
  depend on govuk_app_config

Setting rails_report_rescued_exceptions to false here is better
because it will apply this config to all apps that depend on
govuk_app_config, making the two PRs above redundant and saving us
from making this change in every app.

Setting config.rails_report_rescued_exceptions to false makes having
these exceptions in config.excluded_exceptions redundant.

All the exceptions present in config.action_dispatch.rescue_responses
can be removed from config.excluded_exceptions.

See
https://guides.rubyonrails.org/configuring.html#configuring-action-dispatch
for the list of exceptions in config.action_dispatch.rescue_responses.

Trello card: https://trello.com/c/Ik7ulDXQ/1768-3-upgrade-frontend-to-rails-6