The idea is to use a Layered Asset File-system (modules/assetfs/layered.go)
For example: when there are 2 layers: "custom", "builtin", when access
to asset "my/page.tmpl", the Layered Asset File-system will first try to
use "custom" assets, if not found, then use "builtin" assets.
This approach will hugely simplify a lot of code, make them testable.
Other changes:
* Simplify the AssetsHandlerFunc code
* Simplify the `gitea embedded` sub-command code
---------
Co-authored-by: Jason Song <i@wolfogre.com>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
50a72e7a83