Merge branch 'release/2.1.8'

Merge branch 'release/2.1.7'

Merge branch 'release/2.1.6'

Merge branch 'release/2.1.5'

Merge branch 'release/2.1.4'

Merge branch 'release/2.1.3'

Merge branch 'release/2.1.2'

Merge branch 'release/2.1.1'

Merge branch 'release/2.1.0'

2.0.27: 2.0 sites can selfupgrade to 2.1 (#4299)

A release family was major.minor on every line, built to keep 1.7 sites off 1.8, so a 2.0 site was told it was on the latest version while 2.1.0 was out. From 2.0 on the family is the major version alone; 1.x keeps major.minor. selfupgrade now also prints the next_major hint the server has been sending older sites.