Update default value of applications_settings.max_import_size to 0

What does this MR do?

This MR changes default value of max_import_size in application_settings db table as per suggestion in #251106 (closed)

Migration output

Up

== 20210108161039 UpdateMaxImportSizeDefault: migrating =======================
-- change_column_default(:application_settings, :max_import_size, {:from=>50, :to=>0})
   -> 0.0367s
== 20210108161039 UpdateMaxImportSizeDefault: migrated (0.0368s) ==============

Down

== 20210108161039 UpdateMaxImportSizeDefault: reverting =======================
-- change_column_default(:application_settings, :max_import_size, {:from=>0, :to=>50})
   -> 0.0414s
== 20210108161039 UpdateMaxImportSizeDefault: reverted (0.0445s) ==============

Screenshots (strongly suggested)

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