whatsnew overhaul

[tkknight]

Build

• Removed aggregate_directory.py, and calls to it from .travis.yml
• All unreleased changes can now go directly into latest.rst
• The what's new latest.rst is always built unless a release is made (manual steps, documentation updated)
• The last 2 warnings in the documentation build are now resolved - clean build!

Documentation

• Updated the whats_new_contributions.rst to show the new process
• Update the whatsnew categories (replaced "dependencies" with "internal" so it is more generic)
• Added "Maintainer steps" to the release.rst
• What's new files layout and titles updated to be more useful. 80 char width also applied for readability
• Added a latest.rst.template for easy base lining of a new latest.rst

Misc

• Add html-noplot to the top level Makefile (already in the level below)
• When built on the ReadTheDocs server, all the environment variables will be printed for reference

You can view the build documentation on a test Read The Docs page: https://iris-test-doc.readthedocs.io/en/latest/, specifically:

• https://iris-test-doc.readthedocs.io/en/latest/whatsnew/index.html
• https://iris-test-doc.readthedocs.io/en/latest/whatsnew/latest.html
• https://www-avd/~itkt/iris-docs-dev/src/_build/html/developers_guide/release.html#iris-development-releases-steps
• https://iris-test-doc.readthedocs.io/en/latest/developers_guide/documenting/whats_new_contributions.html

The PR may close #2104 and #3451.

[bjlittle]

@tkknight At a minimum we'd announce this on our scitools-iris twitter feed, don't know if you want to mention that here?.

[tkknight]

this could be added - but this step would need to be added to the maintainer steps too.

[pp-mo]

This is clearly a huge improvement !
I've hit a few typos + ideas, mostly explicit suggested changes.

I really like the release account, and the step-by-step. The account there is much clearer for using a specific example version number.

I'm still not sure we've covered how the process is slightly different for point releases and release candidates, regarding the uses of tags, version strings and documentation builds.
But, I'd like to tackle that separately as this is complicated enough + I'm not sure of my facts right now.