add spam_check_api_key textbox to admin / application_settings / reporting

What does this MR do?

As discussed with @agroleau , @cmiskell and @cwoolley-gitlab and in accordance with @cablett 's comment on !31449 (merged) regarding future iterations for Spam Check's integration with GitLab:

The endpoint is only provided with a user ID at the moment, but that will change in a future iteration as that is developed further.

This MR adds an API key text-box to /admin/application_settings/reporting#js-spam-settings and supporting components so that Spam Check can be provided a token which it can use to authenticate its requests while requesting verdicts from the respective Spam Check service also configured in /admin/application_settings/reporting#js-spam-settings .

spam_verdict_service.rb has not been modified on purpose, since in MR !52385 (merged) we transition from HTTP to GRPC communication. As GRPC-related communication and the actual outbound service request will be moved to lib/gitlab/spamcheck/client.rb, those changes would likely be discarded.

That MR will be updated once this smaller and more simple MR is merged to leverage either the ENV['SPAM_CHECK_API_KEY'] environment variable or Gitlab::CurrentSettings.current_application_settings.spam_check_api_key

This change will likely be made to the following line !52385 (diffs) , complementing the changes already in-place / suggested there.

Screenshots (strongly suggested)

GDK path to the UI changes is http://localhost:3000/admin/application_settings/reporting#js-spam-settings

Database Migration Output

$ bundle exec rails db:migrate RAILS_ENV=development; 
== 20210427125400 AddSpamcheckApiKeyToApplicationSetting: migrating ===========
-- add_column(:application_settings, :encrypted_spam_check_api_key, :text)
   -> 0.0018s
-- add_column(:application_settings, :encrypted_spam_check_api_key_iv, :text)
   -> 0.0007s
== 20210427125400 AddSpamcheckApiKeyToApplicationSetting: migrated (0.0025s) ==

jsalazar@shell:~/gitlab/gitlab-development-kit/gitlab$ bundle exec rails db:migrate:up VERSION=20210427125400
== 20210427125400 AddSpamcheckApiKeyToApplicationSetting: migrating ===========
-- add_column(:application_settings, :encrypted_spam_check_api_key, :text)
   -> 0.0060s
-- add_column(:application_settings, :encrypted_spam_check_api_key_iv, :text)
   -> 0.0010s
== 20210427125400 AddSpamcheckApiKeyToApplicationSetting: migrated (0.0072s) ==

jsalazar@shell:~/gitlab/gitlab-development-kit/gitlab$ bundle exec rails db:migrate:down VERSION=20210427125400
== 20210427125400 AddSpamcheckApiKeyToApplicationSetting: reverting ===========
-- remove_column(:application_settings, :encrypted_spam_check_api_key)
   -> 0.0023s
-- remove_column(:application_settings, :encrypted_spam_check_api_key_iv)
   -> 0.0008s
== 20210427125400 AddSpamcheckApiKeyToApplicationSetting: reverted (0.0032s) ==

Does this MR meet the acceptance criteria?

Conformity

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• Documentation (if required)
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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

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• 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