Issue #3621937: Name a payment account by its uuid, and call it an account

Implements [#3621937 (closed)].

First commit renames; the column and the uuid follow, so that a site migrates once.

getAvailableGateways() and isGatewayAvailable() are gone rather than aliased, so these three land together, in any order but within the same deploy:

• this one
• orchestra !501 (#3623411)
• yoyaku !352 (#3623412)