Adds FAQ about why payments can decrease

This plainly establishes the mechanisms by which donations can decrease.

The aim is to eliminate accusations that anyone involved in the Gratipay
machine can wantonly interfere with someone else's donations unchecked
without prior notice, where unprohibited by law.
Merge pull request #2823 from colindean/faq-payment-failure

Adds FAQ about why payments can decrease
Reorder faq to even up the columns
Match title with anchor
Merge pull request #2828 from gratipay/reorder-faq

reorder FAQ
Bump version to 17.1.5
Bump version to 17.1.5-dev
get rid of version.txt
reformat release.sh
handle branch.sql in release.sh
switch to automated version numbers
bring back version.txt