Releases: Variablize Release Notes

[harding]

HTML site diff (English only): http://dg2.dtrt.org/diff.html

This is a re-do of pull #751 that only implements the parts @saivann and I could both agree on per this comment.

It adds additional variables to the release note files to allow setting the version number and date. The version number is required, and can be used to automatically set the release notes title. The date is optional and can be set hours/days after the release.

Additionally, a Makefile test is added that checks whether the download files exist on the Bitcoin.org server. This can help prevent creating a broken Download page.

Changes in commit 404af8b

• Set variables for all previous releases
• Document variables in README.md
• Update code and templates to use variables
• Add the Download page links to the "dl" CSS class. Also add newlines to make the HTML a bit easier to parse using sed
• Add a new Liquid plugin to print warnings. This is used to print a non-error warning if any release is created without the optional date

Changes in commit b88f6ee

• Renamed the old release files:

  cd _releases
  rename 's/^.*-v//' *
  git add *
  

[saivann]

@harding s/automatical/automatically/

[saivann]

@harding LGTM! I've quickly tested the result and reviewed the code.

[harding]

@saivann thanks! (I'll fix that typo.)

In the absence of critical feedback, this will be merged around 01:00 UTC Friday.

[harding]

Had to rebase to fix Makefile conflict with #776, so merged as 18defab