Fill in release date for 2.12.3

Fill in release date for 2.11.6

Release note for wagtail#6822 in 2.12.2

Release note for wagtail#6822 in 2.11.5

Fetch new translations from Transifex

Fill in release date for 2.11.4

Fill in release date for 2.12

Version bump to 2.12rc1

Fill in release date for 2.11.3

revert to @allcaps's original approach to including robots.txt