Instrument methods used in email diffs

See merge request !4038

Make all of the nested constant instrumentation for core app code work
the same way, add mailer instrumentation, and add instrumentation to the
premailer gem.

This reverts commit 1cc614f2bdd30b4fce35ee9e680f9272b9012978.

It was causing the ActiveJob integration to fail, so unfortunately we'll have to add the gem again.

As described in their Docs: https://docs.getsentry.com/on-premise/clients/ruby/integrations/rails/

Disable Rack Attack if admin disables it in config file

Grack::Auth already checks this variable. These holdouts were not disabled, leading to confusion when debugging a customer issue.

See merge request !3918

Send 'admin emails' weekly, not daily

Daily seems to be to spammy, so let's default to weekly instead.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/15520

See merge request !3923

Grack::Auth already checks this variable. These holdouts were not
disabled, leading to confusion when debugging a customer issue.

These changes were pulled from GitLab EE to support configuring
an alternative API URL than the default https://api.github.com.
In addition, the `verify_ssl` flag allows users to disable SSL cert
checking.

One modification: add a default `args` option if it does not exist
to avoid breaking existing configurations.