Create `IssuableSeverity` model to store a severity level for Incidents

What does this MR do?

After discussion in the following threads #229402 (comment 399736692) and !39857 (comment 399865883) we're changing the implementation idea and create IssuableSeverity model instead. That will allow us to reuse the same table for the different issuable types, such as issues, incidents, bugs (not extracted yet).

Creates IssuableSeverity model to store a severity level for Incidents (and later probably for other issuables).

Closes #238562 (closed) Contributes to #229402 (closed)

Similar to !39857 (closed)

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

Migration output

→ be rails db:migrate
== 20200824124623 CreateIssuableSeverities: migrating =========================
-- create_table(:issuable_severities)
   -> 0.0300s
== 20200824124623 CreateIssuableSeverities: migrated (0.0380s) ================

→ be rails db:rollback
== 20200824124623 CreateIssuableSeverities: reverting =========================
-- drop_table(:issuable_severities)
   -> 0.0525s
== 20200824124623 CreateIssuableSeverities: reverted (0.0525s) ================