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>

web_src/js/features/common-global.js

@@ -12,6 +12,7 @@ import {showTemporaryTooltip} from '../modules/tippy.js';
 import {confirmModal} from './comp/ConfirmModal.js';
 import {showErrorToast} from '../modules/toast.js';
 import {request, POST} from '../modules/fetch.js';
+import 'htmx.org';
 
 const {appUrl, appSubUrl, csrfToken, i18n} = window.config;