Native attachments for Service Desk emails

What does this MR do and why?

This MR changes the default value of the feature flag service_desk_new_note_email_native_attachments from false to true which enalbes native attachments for Service Desk emails.

We want to ship this FF with the 15.10 release and will rollout the FF on gitlab.com. Therefore we need to enable the FF by default so that it's enabled on self-hosted instances by default.

• Rollout issue: [Feature flag] Rollout of `service_desk_new_not... (#386860 - closed)
• Update documentation MR: Change Service Desk native attachments docs for... (!114296 - merged)
• Release post item MR: Native attachments for Service Desk emails (gitlab-com/www-gitlab-com!121161 - merged)

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