Don't do a full page load when clicking the subscribe button (#28871)

- Refactor the form around the subscribe button into its own template
- Use htmx to perform the form submission
- `hx-boost="true"` to prevent the default form submission behavior of a
full page load
- `hx-sync="this:replace"` to replace the current request (in case the
button is clicked again before the response is returned)
  - `hx-target="this"` to replace the form tag with the new form tag
  - `hx-push-url="false"` to disable a change to the URL
  - `hx-swap="show:no-scroll"` to preserve the scroll position
- Change the backend response to return a `<form>` tag instead of a
redirect to the issue page
- Include `htmx.org` in javascript imports

This change introduces htmx with the hope we could use it to make Gitea
more reactive while keeping our "HTML rendered on the server" approach.

# Before


![before](4ec3e81e-4dbf-4338-9968-b0655c276d4c)

# After


![after](8c8841af-9bfe-40b2-b1cd-cd1f3c90ba4d)

---------

Signed-off-by: Yarden Shoham <git@yardenshoham.com>

webpack.config.js

@@ -214,6 +214,7 @@ export default {
       },
       override: {
         'khroma@*': {licenseName: 'MIT'}, // https://github.com/fabiospampinato/khroma/pull/33
+        'htmx.org@1.9.10': {licenseName: 'BSD-2-Clause'}, // "BSD 2-Clause" -> "BSD-2-Clause"
       },
       emitError: true,
       allow: '(Apache-2.0 OR BSD-2-Clause OR BSD-3-Clause OR MIT OR ISC OR CPAL-1.0 OR Unlicense OR EPL-1.0 OR EPL-2.0)',