ENH add optional SiteConfig root level permissions

[andrewandante]

Fixes #113

Offers a SiteConfig-based approach to Root file permissions, in a similar vein to Pages.

Aims to have the same starting point as the current FileDefaultPermissions - that is, view by anyone, edit only by those with the Permission File::EDIT_ALL.

Can be configured with the following yaml (caveat, maybe not all that is needed, IDK):

---
Name: app-assetspermissions
After:
 - assetspermissions
---
SilverStripe\Core\Injector\Injector:
  SilverStripe\Security\PermissionChecker.file:
    class: SilverStripe\Security\InheritedPermissions
    constructor:
      BaseClass: SilverStripe\Assets\File
      CacheService: '%$Psr\SimpleCache\CacheInterface.InheritedPermissions'
    properties:
      DefaultPermissions: '%$SilverStripe\Assets\SiteConfigFilePermissions'
      GlobalEditPermissions:
        - CMS_ACCESS
  SilverStripe\Security\InheritedPermissionFlusher:
    properties:
      Services:
        - '%$SilverStripe\Security\PermissionChecker.file'
SilverStripe\SiteConfig\SiteConfig:
  extensions:
    - SilverStripe\Assets\RootLevelAccessSiteConfigExtension

TODO

• probably remove SiteConfig as a dependency, this shouldn't be invoked unless it is installed
• hide things properly on the page (doing that thing where I need to adjust some React code or all the options are open
• update Readme/Docs to offer this option

[GuySartorelli]

I'm not sure if SiteConfig is the right place for this - it would make more sense to me if instead there was a way to configure this on the root 'folder' in asset admin itself. There's no reason for this to be tied to SiteConfig other than that's where sitetree handles its root permissions, which makes sense in that context because:

1. SiteConfig is already required by the CMS module
2. For pages, those permissions are about access to the site

Asset admin shouldnt have any dependency on SiteConfig, and access to files I think is sufficiently separate from site access that it should be contained withing the asset admin directly.

[andrewandante]

Fair - from a user's perspective, I think that's where I'd expect it to be, but equally we can't put it there without SiteConfig, so I think in this case it should go somewhere else. Thinking here?

Could pop it out like the current "folder" Edit Form, with only a permissions "tab"?

[GuySartorelli]

Yup, that'd make sense to me.
Or perhaps just add an edit icon next to "Files" like when you're editing a folder - since that's essentially what we're doing here.

And then as you say, only have the permissions tab in the form.

[andrewandante]

Oh yeah that's clean, I like that a lot. Now to figure out how to do it 😉

[GuySartorelli]

In the meantime I'll close this PR - please create a new one when/if you have a way forward on the new approach.