Move all integration docs images in a single directory

doc/integration/facebook.md

doc/integration/github.md

doc/integration/gitlab.md

doc/integration/gmail_action_buttons_for_gitlab.md

doc/integration/google.md

doc/integration/img/facebook_api_keys.png

doc/integration/img/facebook_app_settings.png

doc/integration/img/facebook_website_url.png

doc/integration/img/github_app.png

doc/integration/img/gitlab_app.png

doc/integration/img/gmail_action_buttons_for_gitlab.png

doc/integration/img/google_app.png

doc/integration/img/oauth_provider_admin_application.png

doc/integration/img/oauth_provider_application_form.png

...integration/img/oauth_provider_authorized_application.png

...integration/img/oauth_provider_user_wide_applications.png

doc/integration/img/twitter_app_api_keys.png

doc/integration/img/twitter_app_details.png

doc/integration/jira-integration-points.png

doc/integration/oauth_provider.md