Allow customization of the folder view

[dsonck92]

This is going to be a bit of an interesting idea here.

After #146 and #148, it would be interesting if we get tools to customize how a folder is exposed to the user. What I'm thinking about is the ability to set some description and possibly other kinds of layouts. One thing I can imagine is a more photos oriented view (e.g. photo wall) for images, or an embedded full screen video player for video oriented pages.

Probably we could utilize go templates here, giving access to generic metadata fields that can be attached to a directory, and possibly file inclusions like rendering out an Markdown file. Not sure how to deal with the template storage, maybe also inside MinIO. This might have security implications, though the templating engine determines function access.

Otherwise having frontend building blocks also works.

I'm not entirely sure how to exactly enable it. Perhaps a first iteration would be the way Nextcloud supports: you place a text file in the directory, which gets rendered at the top of the page. And metadata attached is just available in a sidebar.