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-lock.json

@@ -26,6 +26,7 @@
         "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",
@@ -6158,6 +6159,11 @@
         "url": "https://github.com/fb55/entities?sponsor=1"
       }
     },
+    "node_modules/htmx.org": {
+      "version": "1.9.10",
+      "resolved": "https://registry.npmjs.org/htmx.org/-/htmx.org-1.9.10.tgz",
+      "integrity": "sha512-UgchasltTCrTuU2DQLom3ohHrBvwr7OqpwyAVJ9VxtNBng4XKkVsqrv0Qr3srqvM9ZNI3f1MmvVQQqK7KW/bTA=="
+    },
     "node_modules/http-proxy-agent": {
       "version": "7.0.0",
       "resolved": "https://registry.npmjs.org/http-proxy-agent/-/http-proxy-agent-7.0.0.tgz",