Don't do a full page load when clicking the follow button (#28872)

- Use htmx to perform the button request
- `hx-headers='{"x-csrf-token": "{{.CsrfToken}}"}'` to authenticate (we
should probably learn to reuse this)
- `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-swap="outerHTML"` to replace the card (as opposed to its inner
content) with the new card that shows the new follower count and button
color
- Change the backend response to return a `<div>` tag (the card) instead
of a redirect to the user page

# Before

![before](86899d15-41c9-42ed-bd85-253b9caac7f8)

# After

![after](59455d96-548c-4a81-a5b0-fab1dc1e87ef)

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

routers/web/user/profile.go

@@ -14,6 +14,7 @@ import (
 	"code.gitea.io/gitea/models/db"
 	repo_model "code.gitea.io/gitea/models/repo"
 	user_model "code.gitea.io/gitea/models/user"
+	"code.gitea.io/gitea/modules/base"
 	"code.gitea.io/gitea/modules/context"
 	"code.gitea.io/gitea/modules/git"
 	"code.gitea.io/gitea/modules/log"
@@ -26,6 +27,10 @@ import (
 	shared_user "code.gitea.io/gitea/routers/web/shared/user"
 )
 
+const (
+	tplProfileBigAvatar base.TplName = "shared/user/profile_big_avatar"
+)
+
 // OwnerProfile render profile page for a user or a organization (aka, repo owner)
 func OwnerProfile(ctx *context.Context) {
 	if strings.Contains(ctx.Req.Header.Get("Accept"), "application/rss+xml") {
@@ -309,8 +314,10 @@ func Action(ctx *context.Context) {
 
 	if err != nil {
 		log.Error("Failed to apply action %q: %v", ctx.FormString("action"), err)
-		ctx.JSONError(fmt.Sprintf("Action %q failed", ctx.FormString("action")))
+		ctx.Error(http.StatusBadRequest, fmt.Sprintf("Action %q failed", ctx.FormString("action")))
 		return
 	}
-	ctx.JSONOK()
+
+	shared_user.PrepareContextForProfileBigAvatar(ctx)
+	ctx.HTML(http.StatusOK, tplProfileBigAvatar)
 }