Move Service Templates UI behind feature flag

To support the removal of Service Templates in %14.0 (&5672 (closed)), we will

• move the Service Template UI behind a feature flag (e.g. :disable_service_templates)
• disable the feature flag by default
• enable the feature flag in %14.0

This approach was discussed in #325196 (comment 532528192)

Feature flag rollout issue: #327436 (closed)