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
Merge branch 'master' with bixly-testing
#663: Save changes in Theme Editor
Merge branch 'master' into bixly-testing
#662: Accept $ in pricing for Events
Merge branch 'master' into bixly-testing