Use a Node-based tools for handling assets

…or how to move a bunch of things from a bunch of different places into
`app/static`.

There are three main reasons not to use Flask Assets:
- It had some strange behaviour like only
- It was based on Ruby SASS, which is slower to get new features than libsass,
  and meant depending on Ruby, and having the SASS Gem globally installed—so
  you’re already out of being a ‘pure’ Python app
- Martyn and I have experience of doing it this way on Marketplace, and we’ve
  ironed out the initial rough patches

The specific technologies this introduces, all of which are Node-based:
- Gulp – like a Makefile written in Javascript
- NPM – package management, used for managing Gulp and its related dependencies
- Bower – also package management, and the only way I can think to have
  GOV.UK template as a proper dependency

…speaking of which, GOV.UK template is now a dependency. This means it can’t be
modified at all (eg to add a global `#content` wrapper), so every page now
inherits from a template that has this wrapper. But it also means that we have a
clean upgrade path when the template is modified.

Everything else (toolkit, elements) I’ve kept as submodules but moved them to a
more logical place (`app/assets` not `app/assets/stylesheets`, because they
contain more than just SASS/CSS).

README.md

@@ -14,32 +14,49 @@ Application to handle the admin functions of the notifications application.
 </ul>
 
 ### Create a virtual environment for this project
-    mkvirtualenv -p /usr/local/bin/python3 notifications-admin 
- 
+```shell
+    mkvirtualenv -p /usr/local/bin/python3 notifications-admin
+```
 
-### GOV.UK frontend toolkit
- The GOV.UK frontend toolkit is a submodule of this project.
- To get the content of the toolkit run the following two commands
- 
-    git submodule init
-    git submodule update
+
+### Building the frontend
+
+You need [Node](http://nodejs.org/) which will also get you [NPM](npmjs.org),
+Node's package management tool.
+```shell
+    brew install node
+```
+
+Most of the frontend dependencies are managed using Git Submodules. Some are
+managed with NPM and Bower. To install or update *all the things*, run
+```shell
+    git submodule init --update
+    npm install
+```
+
+If you want the front end assets to re-compile on changes, leave this running
+in a separate terminal from the app
+```shell
+    npm run watch
+```
 
 ### Running the application:
+```shell
     pip install -r requirements.txt
     ./scripts/bootstrap.sh  
     ./scripts/run_app.sh
+```
 
-Note: the ./scripts/bootstrap.sh script only needs to be run the first time to create the database.
+Note: the ./scripts/bootstrap.sh script only needs to be run the first time to
+create the database.
+
+URL to test app:
 
- url to test app: 
- 
     localhost:6012/helloworld
 
 
 ### Domain model
 
-All the domain models are defined in the [models.py](https://github.com/alphagov/notifications-admin/blob/master/app/models.py) file.
-
-
-
-
+All the domain models are defined in the
+[models.py](https://github.com/alphagov/notifications-admin/blob/master/app/models.py)
+file.