POC Venmo Button

Should be converted to use our SDK before hitting prod.
drop DB rules
Respect Do Not Track header for Segment.io.
Add segment.io testing credentials.
Add segment.io tracking by user id.
Fix segmentio identify functions for anon and auth'd users.
Fix header format for aspen, not django.
Bypass request object modification and check DNT directly from header.
DNT code pushed into util.
Updated privacy policy to mention segment.io.
we don't need a new hook for DoNotTrack
Add mention of DNT to privacy policy.
Move account actions into <td> to fix alignment bugs
Use Venmo's sdk
Merge branch 'master' into add-venmo-button

Conflicts:
	templates/connected-accounts.html
add unique constraint on participants.api_key
Added session_expires check. Resolves #2344
Merge pull request #2377 from ivanmushketyk/session-expires

Check session_expires
merge branch.sql into schema.sql
Bump version to 12.9.7