- Closes https://github.com/go-gitea/gitea/issues/28880
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.
- Add `htmx.js` that imports `htmx.org` and initializes error toasts
- Place `hx-headers='{"x-csrf-token": "{{.CsrfToken}}"}'` on the
`<body>` tag so every request that htmx sends is authenticated
- Place `hx-swap="outerHTML"` on the `<body>` tag so the response of
each htmx request replaces the tag it targets (as opposed to its inner
content)
- Place `hx-push-url="false"` on the `<body>` tag so no changes to the
URL happen in `<form>` tags
- Add the `is-loading` class during request
### Error toasts in action

## Don't do a full page load when clicking the subscribe button
- 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
- Change the backend response to return a `<form>` tag instead of a
redirect to the issue page
### Before

### After

## Don't do a full page load when clicking the follow button
- Use htmx to perform the button request
- `hx-post="{{.ContextUser.HomeLink}}?action=follow"` to send a POST
request to follow the user
- `hx-target="#profile-avatar-card"` to target the card div for
replacement
- `hx-indicator="#profile-avatar-card"` to place the loading indicator
on the card
- Change the backend response to return a `<div>` tag (the card) instead
of a redirect to the user page
### Before

### After

---------
Signed-off-by: Yarden Shoham <git@yardenshoham.com>
Co-authored-by: 6543 <m.huber@kithara.com>
Co-authored-by: Giteabot <teabot@gitea.io>