milestones towards the firefox 58 release

Hi,

as mentioned by Sylvestre, we're looking at using the Developer Edition
channel to push early 58 beta builds before 58 moves to beta proper and 59
development starts, like we did for 57.
You will find a list of 58 milestones here:
https://docs.google.com/document/d/1jPyeW14MyHyQX7kXBU8KN9js
xO2VQjP3GN_VpLiG9YM/edit?ts=59e628ae
This is similar to what happened with 57, with some differences to have a
more "normal" release week.

tl;dr
- November 2nd: 57 merges from mozilla-beta to mozilla-release, and 58
merges to beta for the first time. central will stay at 58, and will keep
regularly merging to beta, for another week
- November 3rd: Developer Edition 58.0b1 goes to build, to be released on
November 7th
- November 6th - November 13th: soft code freeze. Please don't land any
risky patches or enable new features without coordination with release
management during that week.
- November 13th: central becomes 59, and the first "real" 58 beta build
(58.0b3) starts
- from then on it's business as usual

Cheers,
Julien