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.

Co-author: @wxiaoguang 

Close #25096 

The way to fix it in this PR is to change form submit to fetch using
formData, and add flags to avoid post repeatedly.
Should be able to apply to more forms that have the same issue after
this PR.

In the demo below, 'approve' is clicked several times, and then
'comment' is clicked several time after 'request changes' clicked.

After:


https://github.com/go-gitea/gitea/assets/17645053/beabeb1d-fe66-4b76-b048-4f022b4e83a0


Update: screenshots from /devtest

>
![image](https://user-images.githubusercontent.com/2114189/245680011-ee4231e0-a53d-4c2a-a9c2-71ccd98005cc.png)
> 
>
![image](https://user-images.githubusercontent.com/2114189/245680057-9215d348-63d8-406d-8828-17e171163aaa.png)
> 
>
![image](https://user-images.githubusercontent.com/2114189/245680148-89d7b3d1-d7b6-442f-b69e-eadaee112482.png)

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Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

- Follows #23988 
- Fixes: #24074 by removing this key

GitHub's `relative-time` elements allow us to force their rendering to
`auto`, `past`, or `future` tense. We will never show an absolute date
`on ...` in `TimeSince`

## Before

![image](https://user-images.githubusercontent.com/20454870/231735872-048c7bf3-6aa1-4113-929d-75a985c9922c.png)

## After

![image](https://user-images.githubusercontent.com/20454870/231736116-6ad47b63-77f4-4d3f-82a2-ee9a46ba2bd1.png)

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Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

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>

The first step of the plan

* #23290

Thanks to @silverwind for the first try in #15394 . Close #10729 and a
lot of related issues.

The EasyMDE is not removed, now it works as a fallback, users can switch
between these two editors.

Editor list:

* Issue / PR comment
* Issue / PR comment edit
* Issue / PR comment quote reply
* PR diff view, inline comment
* PR diff view, inline comment edit
* PR diff view, inline comment quote reply
* Release editor
* Wiki editor

Some editors have attached dropzone

Screenshots:

<details>


![image](https://user-images.githubusercontent.com/2114189/229363558-7e44dcd4-fb6d-48a0-92f8-bd12f57bb0a0.png)


![image](https://user-images.githubusercontent.com/2114189/229363566-781489c8-5306-4347-9714-d71af5d5b0b1.png)


![image](https://user-images.githubusercontent.com/2114189/229363771-1717bf5c-0f2a-4fc2-ba84-4f5b2a343a11.png)


![image](https://user-images.githubusercontent.com/2114189/229363793-ad362d0f-a045-47bd-8f9d-05a9a842bb39.png)

</details>

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Co-authored-by: silverwind <me@silverwind.io>