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](https://github.com/go-gitea/gitea/assets/20454870/4ec3e81e-4dbf-4338-9968-b0655c276d4c)

# After


![after](https://github.com/go-gitea/gitea/assets/20454870/8c8841af-9bfe-40b2-b1cd-cd1f3c90ba4d)

---------

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

package.json

@@ -25,6 +25,7 @@
     "esbuild-loader": "4.0.2",
     "escape-goat": "4.0.0",
     "fast-glob": "3.3.2",
+    "htmx.org": "1.9.10",
     "jquery": "3.7.1",
     "katex": "0.16.9",
     "license-checker-webpack-plugin": "0.2.1",