Make the Geo OAuth application trusted by default

What does this MR do?

When a Geo secondary node is set up, we create a System OAuth application, which is currently untrusted by default. This means every user that tries to log in on a new secondary will first receive this authorization popup. Considering that Geo is actually "provided by GitLab", the popup seems unnecessary, and we could set the application as trusted by default. This MR makes the Geo OAuth application trusted by default

Migration output

• Up

== 20210212153934 MakeTheGeoOauthApplicationTrustedByDefault: migrating =======
-- execute("UPDATE oauth_applications SET confidential = true, trusted = true WHERE id IN (SELECT oauth_application_id FROM geo_nodes);")
   -> 0.0066s
== 20210212153934 MakeTheGeoOauthApplicationTrustedByDefault: migrated (0.0067s)

• Down

== 20210212153934 MakeTheGeoOauthApplicationTrustedByDefault: reverting =======
== 20210212153934 MakeTheGeoOauthApplicationTrustedByDefault: reverted (0.0000s)

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 issues

Related to #301056 (closed)