Create PlatformContext and Change isAmp to be a constant platform

[ChrisBAshton]

Is your feature request related to a problem? Please describe.
Arising from the meeting in #884: we've identified that one day we may have to support more than just 'canonical' and 'AMP'.

To prevent lock-in to binary platform delivery, we should phase out our use of the isAmp boolean and instead refer to a platform string which may have the value 'canonical', 'AMP' or something else in the future.

Notes from Jim:

So at the moment the assumption is that FIA and Apple News stay with Syndication
However we may want to render a syndicatable HTML version of the article - this is something Bogdan and I have discussed a little
so it does make sense to consider a world where we are rendering more than two variants

Describe the solution you'd like
Create the PlatformContext, and use PlatformContex.Provider in the Article container & PlatformContainer.Consumer in the Image container. A POC can be seen here: #878.

Rather than passing isAmp as props, pass platform via the context. Almost everywhere in our code where we refer to isAmp, we should make a change.

That said, in containers which need AMP logic, there's nothing stopping us from doing isAmp = platform === 'amp' and then referring to isAmp in the rest of the file.

Describe alternatives you've considered
The alternative is passing the platform as a prop down nested components - quickly gets messy.

Another alternative is some centralised state (eg Redux), but this isn't necessary at this time.

The alternative is to keep things the way they are - but could become a maintenance nightmare if that 3rd platform becomes a requirement in the future! Making the change to platform means we'd have to make changes to a minimal number of files.

Testing notes
Unit tests should be sufficient.

• Initially labelled with "Refinement needed"

[ChrisBAshton]

To be merged with #928.