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>

routers/web/repo/issue_watch.go

@@ -8,10 +8,15 @@ import (
 	"strconv"
 
 	issues_model "code.gitea.io/gitea/models/issues"
+	"code.gitea.io/gitea/modules/base"
 	"code.gitea.io/gitea/modules/context"
 	"code.gitea.io/gitea/modules/log"
 )
 
+const (
+	tplWatching base.TplName = "repo/issue/view_content/watching"
+)
+
 // IssueWatch sets issue watching
 func IssueWatch(ctx *context.Context) {
 	issue := GetActionIssue(ctx)
@@ -52,5 +57,7 @@ func IssueWatch(ctx *context.Context) {
 		return
 	}
 
-	ctx.Redirect(issue.Link())
+	ctx.Data["Issue"] = issue
+	ctx.Data["IssueWatch"] = &issues_model.IssueWatch{IsWatching: watch}
+	ctx.HTML(http.StatusOK, tplWatching)
 }