Updates to UTM param persistence, localStorage/cookie migration, session recording

• UTM parameters are no longer persisted as superproperties by default. Mixpanel analyses now have attribution support that does not require client-side persistence of these properties. To opt in to the previous behavior, use initialization option {stop_utm_persistence: false}.
• localStorage->cookie migration support: when switching an implementation from localStorage persistence to cookie persistence (to support cross-subdomain tracking), the SDK will now automatically copy any existing superproperties from localStorage into the new superprop cookie. This migration behavior already existed in the opposite direction (going from cookie to localStorage).
• The initialization options record_block_class, record_block_selector, and record_mask_text_class offer finer-grained control over elements to block in session recording, and provide stricter defaults.
• New method mixpanel.get_session_recording_properties() exposes Replay ID property for tagging events controlled by other client-side SDKs such as Segment or mParticle.