Sanitize slashes from title

[tobbbe]

Remove slashes from title. A slash will cause many git UIs to nest the branch, which probably isnt wanted?

• Extension version: all
• VSCode Version: all
• OS: all

Steps to Reproduce:

1. Create an issue that includes a slash ("/").
2. Click "Start work on issue..." in vscode sidebar.
3. A branch is created that will include the slash