Replace #25446, fix#25438
All "cancel" buttons which do not have "type" should not submit the
form, should not be triggered by "Enter".
This is a complete fix for all modal dialogs.
The major change is "modules/aria/modal.js", "devtest" related code is
for demo/test purpose.
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
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Co-authored-by: Jason Song <i@wolfogre.com>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
