ability to choose source branch when creating a branch from an issue

Description

Right now it looks like creating a new branch from an issue will use the default branch as the source branch

Proposal

I propose to add a select box that will allow us to choose which branch to use as the source for the new branch when creating the new branch from an issue

Links / references

https://docs.gitlab.com/ee/user/project/repository/web_editor.html#sts=Create%20a%20new%20branch%20from%20an%20issue

clone of this issue at gitlab-ce
https://gitlab.com/gitlab-org/gitlab-ce/issues/22264