Create IncidentData model to store incident related info

What does this MR do?

Closes #238562 (closed)

Creates IncidentManagement::IncidentData model to store incident related info. (Starting with severity)

Follows "Creating a new table when we have two foreign keys" example: https://docs.gitlab.com/ee/development/migration_style_guide.html#examples

Screenshots

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

rails db:migrate output

== 20200819132514 CreateIncidentData: migrating ===============================
-- table_exists?(:incident_data)
   -> 0.0012s
-- create_table(:incident_data)
   -> 0.0350s
== 20200819132514 CreateIncidentData: migrated (0.0363s) ======================

== 20200819134741 AddForeignKeyToProjectIdOnIncidentData: migrating ===========
-- add_foreign_key(:incident_data, :projects, {:column=>:project_id, :on_delete=>:cascade})
   -> 0.0112s
== 20200819134741 AddForeignKeyToProjectIdOnIncidentData: migrated (0.0197s) ==

== 20200819135243 AddForeignKeyToIssueIdOnIncidentData: migrating =============
-- add_foreign_key(:incident_data, :issues, {:column=>:issue_id, :on_delete=>:cascade})
   -> 0.0039s
== 20200819135243 AddForeignKeyToIssueIdOnIncidentData: migrated (0.0052s) ====