point to FAQ instead of GitHub
Merge pull request #2720 from gratipay/link-to-faq-not-github

Point to FAQ instead of GitHub
Bump version to 16.0.6
Bump version to 16.0.6-dev
simplify functions that take an optional cursor
add a clear_takes() method to Participant

also fixes the 4th item of #2630 (was #1920)
remove obsolete reference in comment
zero out giving/receiving/etc amounts in Participant.archive()
add a fake payday implemented in SQL
make get_tip_to and set_tip_to return a dict instead of only the amount
drop the goal_summary view

We don't use it and its definition is obsolete.
compute giving and receiving amounts more accurately

fixes #1127
fix tests
fix condition
rename function
make tests more thorough