Use global variable for CDN root directory

[diarmidmackenzie]

Description:

As per #5119

Changes proposed:

Replace hardcoded instances of https://cdn.aframe.io/ with a global variable AFRAME_ROOT_CDN.

Based on @vincentfretin's suggestion here

[vincentfretin]

Thanks for doing this work.

[vincentfretin]

The text in https://github.com/aframevr/aframe/blob/master/docs/introduction/faq.md#can-i-use-a-frame-offline-or-self-hosted would also need to be modified.

[diarmidmackenzie]

The text in https://github.com/aframevr/aframe/blob/master/docs/introduction/faq.md#can-i-use-a-frame-offline-or-self-hosted would also need to be modified.

I'd like to handle that as a separate PR.

I don't think the text as it stands needs to change for this PR to be OK - everything it says is still true, even after this code change.

More work needed to fully solve #5119, but I'd like to tackle this one bit at a time.

[nightgryphon]

it also be cool to add an assets archive for loacal deployment. Including fonts, controllers models etc. I has to manualy figure out which files are required for local deployment and download one by one...

[vincentfretin]

@nightgryphon You can clone the assets repo actually, I commented about it here #5119 (comment)

[nightgryphon]

@vincentfretin Ohh! I've missed that repo. It can be a good idea to add a comment line with repo url near the constant and later in the docs. Also it is good to note which folders are actually required.

[dmarcos]

Thanks!