Polish up the ScenariosUIBlock in dfs_default for ACSF et al.

[saltednut]

In some instances (e.g. ACSF), we see that images from default_content_deploy do not land during site installation when a scenario is either a dependent, or enabled via a hook after site-install finishes. In both cases, we see broken hero images from dfs_default - our default scenario. I spent some time on work-arounds but it seems to be related to platform architecture, which got me thinking...

I propose we clean this up a bit to make it more consistent by getting rid of the hero block on the homepage altogether. It would be better for all platforms to ship a single stock image with the dfs_default scenario, and use CSS on the ScenariosUIBlock to add a background image and get rid of the gross negative margin stuff we're currently doing. We do want to center everything still, both horizontally and vertically. It needs to support a grid of more than just two scenarios eventually.

See: https://github.com/acquia/df/blob/8.x-4.x/modules/dfs/dfs_default/src/Plugin/Block/ScenariosUIBlock.php#L34 And: https://github.com/acquia/df/blob/8.x-4.x/modules/dfs/dfs_default/css/scenarios_list_default.css#L2

[saltednut]

I did a minor fix here. We can leave this open for now and revisit it after we get more feedback.

The main changes I made were:

1. Changing permissions so the scenarios grid can be seen but the button to "Install" is hidden from anon users
2. Removed the hero on the homepage and deleted it from the default content
3. Changed block-loaded CSS to add the bg image to the parent wrapper and themed accordingly
4. Changed image to something more calming and less complicated so users are less intimidated