release-helpers: Ease release preparations

Some of these are rough cut but let's have them in a repo rather than hidden away on a single machine:

• prep-rc helps to prepare a release candidate branch
• update-ci updates the CI scripts
• announce-string-freeze builds the release announcement for discourse

The other scripts are helpers but can also be invoked on their own.

The prep-rc script is the one that will likely change quiet a bit over time as we also want to do releases with it at some point.