Add unscoped_issue_list_api app setting

What does this MR do?

Initial review already done in a confidential MR https://gitlab.com/gitlab-org/security/gitlab/-/merge_requests/1399

unscoped_issue_list_api application setting is added. true by defauls so it's not a breaking change. Setting this one to false will give an error on GET /api/v4/issues_statistics?scope=all and GET /api/v4/issues?scope=all if no additional filtering is provided. e.g. assignee_id, assignee_username.

Currently in gitlab.com /api/v4/issues_statistics?scope=all timeouts.

DB Migration

UP

== 20210531200126 AddUnscopedIssueListApiToApplicationSettings: migrating =====
-- add_column(:application_settings, :unscoped_issue_list_api, :boolean, {:null=>false, :default=>true})
   -> 0.0043s
== 20210531200126 AddUnscopedIssueListApiToApplicationSettings: migrated (0.0126s)

DOWN

== 20210531200126 AddUnscopedIssueListApiToApplicationSettings: reverting =====
-- remove_column(:application_settings, :unscoped_issue_list_api)
   -> 0.0021s
== 20210531200126 AddUnscopedIssueListApiToApplicationSettings: reverted (0.0104s)

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• 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 #330519