MVC: add new branch rule (very basic solution)

Problem

Overview Branch rules in its current unreleased state (as of 15.8) does not fully live up to its promise in the tag line: Define rules for who can push, merge, and the required approvals for each branch. as it is missing the functionality to Add a new branch rule

Proposal

To quickly fix this and to contribute to our Q1 OKR https://gitlab.com/gitlab-com/gitlab-OKRs/-/work_items/122585230+ we shall:

Add a Add branch rule button to the branch rules overview that opens a modal explaining that it can't be done yet and one would have to create a branch protection first.

(Long-term the experience will be improved with [MVC] Branch Rules Editing (&8075 - closed).)