Add user blocking (#29028)

Fixes #17453

This PR adds the abbility to block a user from a personal account or
organization to restrict how the blocked user can interact with the
blocker. The docs explain what's the consequence of blocking a user.

Screenshots:


![grafik](https://github.com/go-gitea/gitea/assets/1666336/4ed884f3-e06a-4862-afd3-3b8aa2488dc6)


![grafik](https://github.com/go-gitea/gitea/assets/1666336/ae6d4981-f252-4f50-a429-04f0f9f1cdf1)


![grafik](https://github.com/go-gitea/gitea/assets/1666336/ca153599-5b0f-4b4a-90fe-18bdfd6f0b6b)

---------

Co-authored-by: Lauris BH <lauris@nix.lv>

routers/api/v1/user/follower.go

@@ -5,6 +5,7 @@
 package user
 
 import (
+	"errors"
 	"net/http"
 
 	user_model "code.gitea.io/gitea/models/user"
@@ -221,11 +222,17 @@ func Follow(ctx *context.APIContext) {
 	// responses:
 	//   "204":
 	//     "$ref": "#/responses/empty"
+	//   "403":
+	//     "$ref": "#/responses/forbidden"
 	//   "404":
 	//     "$ref": "#/responses/notFound"
 
-	if err := user_model.FollowUser(ctx, ctx.Doer.ID, ctx.ContextUser.ID); err != nil {
-		ctx.Error(http.StatusInternalServerError, "FollowUser", err)
+	if err := user_model.FollowUser(ctx, ctx.Doer, ctx.ContextUser); err != nil {
+		if errors.Is(err, user_model.ErrBlockedUser) {
+			ctx.Error(http.StatusForbidden, "FollowUser", err)
+		} else {
+			ctx.Error(http.StatusInternalServerError, "FollowUser", err)
+		}
 		return
 	}
 	ctx.Status(http.StatusNoContent)