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/repo/pull.go

@@ -362,6 +362,8 @@ func CreatePullRequest(ctx *context.APIContext) {
 	// responses:
 	//   "201":
 	//     "$ref": "#/responses/PullRequest"
+	//   "403":
+	//     "$ref": "#/responses/forbidden"
 	//   "404":
 	//     "$ref": "#/responses/notFound"
 	//   "409":
@@ -510,9 +512,11 @@ func CreatePullRequest(ctx *context.APIContext) {
 	if err := pull_service.NewPullRequest(ctx, repo, prIssue, labelIDs, []string{}, pr, assigneeIDs); err != nil {
 		if repo_model.IsErrUserDoesNotHaveAccessToRepo(err) {
 			ctx.Error(http.StatusBadRequest, "UserDoesNotHaveAccessToRepo", err)
-			return
+		} else if errors.Is(err, user_model.ErrBlockedUser) {
+			ctx.Error(http.StatusForbidden, "BlockedUser", err)
+		} else {
+			ctx.Error(http.StatusInternalServerError, "NewPullRequest", err)
 		}
-		ctx.Error(http.StatusInternalServerError, "NewPullRequest", err)
 		return
 	}
 
@@ -630,6 +634,8 @@ func EditPullRequest(ctx *context.APIContext) {
 		if err != nil {
 			if user_model.IsErrUserNotExist(err) {
 				ctx.Error(http.StatusUnprocessableEntity, "", fmt.Sprintf("Assignee does not exist: [name: %s]", err))
+			} else if errors.Is(err, user_model.ErrBlockedUser) {
+				ctx.Error(http.StatusForbidden, "UpdateAssignees", err)
 			} else {
 				ctx.Error(http.StatusInternalServerError, "UpdateAssignees", err)
 			}