Add API Fuzzing plan limits db column

What does this MR do?

Add a new column ci_max_artifact_size_api_fuzzing to the plan_limits database table. This field is required to add support for the new API Fuzzing report type.

Addresses #258203 (closed)

This MR follows the guidelines for adding new plan limits.

Default plan limits for security reports are currently 0.

Migration up:

$ bundle exec rake db:migrate
(in /home/mike/gdk/gitlab)
== 20200930132319 AddApiFuzzingToPlanLimits: migrating ========================
-- add_column(:plan_limits, "ci_max_artifact_size_api_fuzzing", :integer, {:default=>0, :null=>false})
   -> 0.0027s
== 20200930132319 AddApiFuzzingToPlanLimits: migrated (0.0028s) ===============

Migration down:

$ bundle exec rake db:migrate:down VERSION=20200930132319
(in /home/mike/gdk/gitlab)
== 20200930132319 AddApiFuzzingToPlanLimits: reverting ========================
-- remove_column(:plan_limits, "ci_max_artifact_size_api_fuzzing", :integer, {:default=>0, :null=>false})
   -> 0.0014s
== 20200930132319 AddApiFuzzingToPlanLimits: reverted (0.0060s) ===============

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