Replace Errbit/Airbrake with Sentry

[javierm]

Background

We've got experimental support for Errbit to receive information about exceptions taking place on production environments.

However, Errbit requires MongoDB, but MongoDB changed it license in version 4.0 and is no longer distributed by the most common GNU/Linux distributions.

Objectives

• Use a tool to track exceptions that doesn't depend on software with non-free licenses

Definition of Done

• The Gemfile and Gemfile_custom files are updated to depend on Sentry and suggest Airbrake (the opposite of what we're doing now)
• The config/initializers/errbit.rb file is replaced by a new config/initializers/sentry.rb file
• The config/secrets.yml.example file is updated accordingly
• The Consul Democracy Installer installs Sentry instead of Errbit (just like with Errbit, a flag can enable/disable the installation of Sentry) and Sentry is automatically integrated with the Consul Democrac application, like Errbit is now