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](4ed884f3-e06a-4862-afd3-3b8aa2488dc6)


![grafik](ae6d4981-f252-4f50-a429-04f0f9f1cdf1)


![grafik](ca153599-5b0f-4b4a-90fe-18bdfd6f0b6b)

---------

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

routers/api/v1/repo/fork.go

@@ -149,6 +149,8 @@ func CreateFork(ctx *context.APIContext) {
 	if err != nil {
 		if errors.Is(err, util.ErrAlreadyExist) || repo_model.IsErrReachLimitOfRepo(err) {
 			ctx.Error(http.StatusConflict, "ForkRepository", err)
+		} else if errors.Is(err, user_model.ErrBlockedUser) {
+			ctx.Error(http.StatusForbidden, "ForkRepository", err)
 		} else {
 			ctx.Error(http.StatusInternalServerError, "ForkRepository", err)
 		}