add a constraint on transfers for (amount > 0)
fix take_over() to not create null transfers
delete existing null transfers
only enforce the new constraint on future rows

We can't enforce it on all existing rows because we have an anomalous row that we can't delete.
Show link to communities to people who aren't giving
Add link to widgets on homepage
show onboarding steps to all inactive users
use SESSION_TIMEOUT instead of 15 seconds
skip tips that are rounded down to zero

should fix https://app.getsentry.com/gratipay/gratipay-com/group/30338239/
Merge pull request #2723 from gratipay/positive-transfers-constraint

Only allow strictly positive transfers
Merge pull request #2747 from gratipay/fix-final-gift

Fix distribute_balance_as_final_gift()
merge branch.sql into schema.sql
Bump version to 16.0.10
Bump version to 16.0.10-dev
clearer indentation
add a sign out link floating on the right above the title
Merge pull request #2739 from gratipay/small-homepage-changes

small, quick changes to homepage before redesign
Fixed references to PRs in schema.sql