Enabling caching of classes slows start-up time because all controllers
are loaded at initialization, but it reduces memory and load because files
are not reloaded with every request. For example, caching is not necessary
for loading database migrations but useful for handling Knapsack specs.

Addresses gitlab-org/gitlab-ee#2162

The idea is that after each feature spec example, we block all incoming
requests at the Rack level, go to the 'about:blank' page, and wait until
the current requests reach 0.

This should solve the problem where a request would end after database
cleaner performed the database truncation. The problem was that a GET
request can still lead to records creation (e.g. namespaces or routes).

Signed-off-by: Rémy Coutable <remy@rymai.me>

quiet_assets has been seemingly abandoned, and now sprockets-rails has the feature built-in!

From this PR: https://github.com/rails/sprockets-rails/pull/355

Sent emails in the development environment will no longer automatically
be opened in the browser. If a sent email needs to be viewed, it can be
found at the `/rails/letter_opener` route.

`config.assets.compress` needed to be explicitly enabled.

Follow-up to !3544. Resolves #14344.