Add parallel persistence for author_name on AuditEvent

What does this MR do?

As part of the wider AuditEvent refactoring, it does the following:

• Creates a new database column (author_name) on AuditEvent.
• Reroutes author_name to be stored in both the new column and the existing details hash.
• Updates specs to take this in to account
• Reads from the new database column by default, and falls back to the details hash for older records.

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

Database Review

Migration

== 20200623090030 AddAuthorNameToAuditEvent: migrating ========================
-- column_exists?(:audit_events, :author_name)
   -> 0.0010s
-- add_column(:audit_events, :author_name, :text)
   -> 0.0003s
-- transaction_open?()
   -> 0.0000s
-- execute("ALTER TABLE audit_events\nADD CONSTRAINT check_83ff8406e2\nCHECK ( char_length(author_name) <= 255 )\nNOT VALID;\n")
   -> 0.0003s
-- execute("ALTER TABLE audit_events VALIDATE CONSTRAINT check_83ff8406e2;")
   -> 0.0736s
== 20200623090030 AddAuthorNameToAuditEvent: migrated (0.0781s) ===============

Rollback

== 20200623090030 AddAuthorNameToAuditEvent: reverting ========================
-- remove_column(:audit_events, :author_name)
   -> 0.0013s
== 20200623090030 AddAuthorNameToAuditEvent: reverted (0.0014s) ===============

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 #220318 (closed)