Refactor target_details from AuditEvent details

What does this MR do?

This MR adds a target_details column to AuditEvent and persists the value of details[:target_details] to the target_details column when an AuditEvent is created (or updated, not that it should ever be updated as AuditEvents should be immutable.)

Database

Migrate

== 20200710120500 AddTargetDetailsToAuditEvent: migrating =====================
-- add_column(:audit_events, :target_details, :text)
   -> 0.0009s
== 20200710120500 AddTargetDetailsToAuditEvent: migrated (0.0064s) ============

Rollback

== 20200710120500 AddTargetDetailsToAuditEvent: reverting =====================
-- remove_column(:audit_events, :target_details)
   -> 0.0006s
== 20200710120500 AddTargetDetailsToAuditEvent: reverted (0.0018s) ============

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

Mentions #220325 (closed)