Add ApplicationSetting snowplow_iglu_registry_url

What does this MR do?

Adds an ApplicationSetting for specifying a custom snowplow_iglu_registry_url.

This new setting will be set to the Iglu Registry URL: https://gitlab-org.gitlab.io/iglu from GitLab project https://gitlab.com/gitlab-org/iglu/.

The Iglu Schema Registry URL can then be used for Contextual PageView tracking events that are being implemented in https://gitlab.com/gitlab-org/telemetry/issues/110 as well as future and current front-end and back-end SnowPlow tracking custom events that require a custom schema.

Screenshots

Migration output

== 20191010174846 AddSnowplowIgluRegistryUrlToApplicationSettings: migrating ==
-- add_column(:application_settings, :snowplow_iglu_registry_url, :string, {:limit=>255})
   -> 0.0013s
== 20191010174846 AddSnowplowIgluRegistryUrlToApplicationSettings: migrated (0.0013s) 

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance 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

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