Sprint -> Release

For #7828 and https://trello.com/c/atMTdx6l/1313-brackets-7828-should-be-consistent-about-release-vs-sprint-issue

Replaces most of the occurences of `Sprint` with `Release`, maybe even too many.

The only ones I left in place were some unit tests and the non-English translations (to be translated).
Should it be lowercase?