Check incident_management_settings before sending alert email

What does this MR do?

The current behavior is: An email is always sent to developers when an alert is received from alertmanager.

The new behavior:

An email should be sent only if the appropriate checkbox is not unticked in the Operations Settings. Note that the settings are stored in the ProjectIncidentManagementSettings model. This means that the email should be sent if any of the following is true:

1. The project has the incident_management feature_flag and license enabled but does not have an entry in the ProjectIncidentManagementSettings model. This is to keep the default behavior of sending emails to developers.
2. The project has the incident_management feature_flag and license enabled and ProjectIncidentManagementSettings.send_email is set to true.
3. The project has the incident_management feature_flag or license disabled. This is also to keep the current behavior of sending emails to developers.

Note that if the incident_management license is disabled, but the incident_management feature flag is enabled, the alert email will not be sent out.

This MR changes the NotifyService to respect the ProjectIncidentManagementSettings.send_email setting.

The new settings form that is being added to the Operations Settings page:

What are the relevant issue numbers?

#4925 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?
• Security reports checked/validated by reviewer