Removing the estimate attribute from invoices when exporting

- Estimate is no longer an attribute on the Invoice model
Version bump to 5.1.218
Documentation updates for remote installation from feedback from @jbolles
Trap the IntegrityError

The IntegrityError could happen in the race condition when two processes adding the same user to the same group.
#661: Uncheck Expires as default for Stories add
#660: Automatically change end date and time in Event add
#663: Save changes in Theme Editor
#662: Accept $ in pricing for Events
Merging 452_payment_required_per_pricing branch
Add username option for login page redirect.
tendenci base fields schema update for photo_set - creator, owner, entity..
added exif_data field to Image model
removed duplicates and unused imports
save exif_data to image table
added filter exif_to_date to convert exif date string to a datetime object
display exif info on photo details
management command to populate the exif_data field
replaced map with a location string
version bump
trap AttributeError