Refactor resetting secret fields in integrations

Summary

Some integrations reset their "secret" fields (type: 'password') when certain "exposing" fields are changed (e.g. URLs), to avoid leaking secrets to unauthorized parties:

• https://gitlab.com/gitlab-org/gitlab/blob/49a4911e011419a40b37d0aa18d7f5ccc4048e04/app/models/integrations/bamboo.rb#L23
• https://gitlab.com/gitlab-org/gitlab/blob/49a4911e011419a40b37d0aa18d7f5ccc4048e04/app/models/integrations/jenkins.rb#L21
• https://gitlab.com/gitlab-org/gitlab/blob/49a4911e011419a40b37d0aa18d7f5ccc4048e04/app/models/integrations/jira.rb#L67
• https://gitlab.com/gitlab-org/gitlab/blob/49a4911e011419a40b37d0aa18d7f5ccc4048e04/app/models/integrations/teamcity.rb#L33

Improvements

• Extract this behaviour into a shared concern.
• Add this behaviour to other integrations with secret fields:
  • app/models/integrations/datadog.rb
  • app/models/integrations/zentao.rb

Related links

• !66972 (019f5d16) was a draft to introduce a concern.
• #336004 will need information about the "secret" and "exposing" fields to automatically update the UI.