Breaking: v6 with backward compatibility

[oliverfoster]

fixes https://github.com/adaptlearning/adapt_framework/issues/2526 fixes https://github.com/adaptlearning/adapt_framework/issues/2363

original issues https://github.com/adaptlearning/adapt_framework/issues/3044 original prototype https://github.com/adaptlearning/adapt_framework/pull/3324

What's changed?

1. We have NPM install support for plugins via our normal registry server. The server is publically available for those who wish to host their own registries. A registry server can be setup to fallback to any other public npm registry. The default server fallback registry is to the npm public registry but it is possible to chain to the adapt registry which then chains to the npm one. Any plugins registered in the server should be installable via npm using the registry at https://adapt-bower-repository.herokuapp.com/npm/. With:

//.npmrc
registry=https://adapt-bower-repository.herokuapp.com/npm/

2. We've gotten rid of src/course in favour of build/course
3. It will be possible to use (most/some) npm modules in courses moving forwards (tbc as to limitations).
4. All existing plugins should work without amendment

Testing

1. Run the following commands

git clone https://github.com/adaptlearning/adapt_framework v6-test
cd v6-test
git checkout issue/v6-simple
cd src/
npm install
cd ../
npm install
grunt dev

2. Once built, cancel the dev task with ctrl+c, then run

grunt server

3. Please have a look around and post any comments on this pr

Todo

• SCHEMAS: The grunt tasks do not yet apply defaults from the new AAT schemas, the old AAT schemas will not work with v6 without sustantial work but the new AAT schema styles should be able to do what's necessary, they have yet to be cleaned up and trimmed. We need to decide if v6 will support the classic AAT / vice-versa.
• adapt-cli is not yet compatible with v6 but the adapt install command etc will be made v6 compatible https://github.com/adaptlearning/adapt-cli/issues/175
• grunt build includes do not yet work with dependent extensions, such that if resources is included but drawer is not, drawer will not yet be added to the construction phase