need a way to add notes that don't disappear when a new build is started

[bhearsum]

During the 59.0 cycle we had a few instances of needing to add some notes prior to a release being started that we needed to pay attention to after the release started. For example, Fennec 59.0 (https://github.com/mozilla-releng/releasewarrior-data/blob/master/inflight/fennec/fennec-release-rc-59.0rc.md) had a prerequisite to remind us to do an extra thing when shipping RC, but it stopped showing up on "release status" as soon as we started a new build - which meant we didn't remember.

Fixing this could be as simple as "don't hide prereqs when new builds start".

[escapewindow]

I think we should have a notes array for the entire release keep prereqs over new builds, and a notes array per issue.