Add severity icon for list view

What does this MR do?

A tiny MVC for #214528 (closed)

The goal of this MR is to add the smallest amount of code possible to generate the correct severity icons for only the list view. Given the list view itself is unfinished from a styling and functional perspective, we are only worried about the severity icon in this MR.

We now support all the icons here with the correct colors:

https://gitlab.com/gitlab-org/gitlab/uploads/fec29ee36300c5bd23a9d3b0d2a28e14/Screen_Shot_2020-04-01_at_2.27.31_PM.png

🇺🇦 Alerts section is behind feature flag: make sure to run Feature.enable(:alert_management_minimal) in rails console

When enabled, navigate to "Your project" => Operations => Alerts

To generate sample alerts data you can run

Project.all.each do |project|
  (1..5).each do |i|
    AlertManagement::Alert.create!(started_at: i.hours.ago, title: "Alert #{i}", severity: AlertManagement::Alert.severities.keys.sample, hosts: ['192.168.1.1'], monitoring_tool: 'Grafana', service: 'gitlab', project: project)
  end
end

It will add alerts to each of your projects

Screenshots

##Further Work

1. Mobile / tablet styling will be handled in a separate MR
2. Status will be handled in a separate MR, for the initial testing purpose it is set as null

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team