Change default value for issues_create_limit

Relates to #223718 (closed)

The column issues_create_limit was added to the table application_settingsin !28129 (merged) with a default value of 300. It was decided to change this default value to 0 so this limit is disabled by default.

What does this MR do?

• Adds migration to change the default value in application_settings.issues_create_limit from 300 to 0

Migrations

20200710105332

db:migrate:up

bin/rails db:migrate:up VERSION=20200710105332 
== 20200710105332 ChangeIssuesCreateLimitDefault: migrating ===================
-- change_column_default(:application_settings, :issues_create_limit, {:from=>300, :to=>0})
   -> 0.0323s
== 20200710105332 ChangeIssuesCreateLimitDefault: migrated (0.0365s) ==========

db:migrate:down

bin/rails db:migrate:down VERSION=20200710105332
== 20200710105332 ChangeIssuesCreateLimitDefault: reverting ===================
-- change_column_default(:application_settings, :issues_create_limit, {:from=>0, :to=>300})
   -> 0.0363s
== 20200710105332 ChangeIssuesCreateLimitDefault: reverted (0.0405s) ==========

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 #223718 (closed)