Refactor target_type in AuditEvent

What does this MR do?

As part of the wider AuditEvent refactoring work. This MR:

• Adds target_type:text column to AuditEvent
• Writes target_type to this new column as well as maintaining the original behaviour.
• target_type is not displayed on the frontend but it is logged to the output JSON log file.
• Fills in some of the missing specs for AuditEventService which will hopefully make refactoring easier in the future.

Database Review

Migrate

== 20200706092954 AddTargetTypeToAuditEvent: migrating ========================
-- add_column(:audit_events, :target_type, :text)
   -> 0.0011s
== 20200706092954 AddTargetTypeToAuditEvent: migrated (0.0062s) ===============

== 20200709115716 AddTextLimitToAuditEventTargetType: migrating ===============
-- transaction_open?()
   -> 0.0000s
-- execute("ALTER TABLE audit_events\nADD CONSTRAINT check_82294106dd\nCHECK ( char_length(target_type) <= 255 )\nNOT VALID;\n")
   -> 0.0028s
-- execute("ALTER TABLE audit_events VALIDATE CONSTRAINT check_82294106dd;")
   -> 0.0501s
== 20200709115716 AddTextLimitToAuditEventTargetType: migrated (0.0562s) ======

Rollback

== 20200709115716 AddTextLimitToAuditEventTargetType: reverting ===============
-- execute("ALTER TABLE audit_events\nDROP CONSTRAINT IF EXISTS check_82294106dd\n")
   -> 0.0009s
== 20200709115716 AddTextLimitToAuditEventTargetType: reverted (0.0063s) ======

== 20200706092954 AddTargetTypeToAuditEvent: reverting ========================
-- remove_column(:audit_events, :target_type)
   -> 0.0018s
== 20200706092954 AddTargetTypeToAuditEvent: reverted (0.0019s) ===============

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

Related to #220322 (closed)