CMS5 support

[thats4shaw]

Adds CMS5 support but also allowing CMS4 backwards compatibility.

[izzudin96]

Hi @thats4shaw , can the version be

"silverstripe/framework": "^4.0 || ^5.0",
"silverstripe/admin": "^1.0 || ^2.0",
"silverstripe/siteconfig": "^4.0 || ^5.0"

so that it can support both SS4 and SS5. Thanks :)

[thats4shaw]

@izzudin96 sounds sensible as there hasn't been a tagged release for a while.

Updated.