Improve build and release process

[ianmjones]

Resolves DELI-1149

Things done:

• Minified CSS and JS assets are no longer stored in git, and have been added to .gitignore.
• Minified CSS and JS assets are now only generated when their source is touched or the minified asset is missing.
• The pot language file is only regenerated when PHP source is touched.
• If the node_modules directory is missing, it is automatically created via yarn install when any kind of make target that requires grunt is referenced.
• Running make on its own now builds any assets or updates the pot file as necessary.
• Running make package or make zip are still available, and still effectively do the same thing.
• A make clean target has been added to remove any ephemeral assets or build files.
• A make clean-all target has been added does the same as clean but also remove node_modules.
• The package-lock.json file that is not needed as we use yarn has been removed.
• The package.json file's version entry has been updated to stay in sync with the current version of the plugin.
• The old out of date README.md has been removed, the always up to date README.txt will suffice for now (we may add a less likely to go out of date README.md in the future).