Enable creating mirrors with a different base branch

### Is your feature request related to a problem?

Some projects make use of multiple branches for different purposes (ex. gh-pages for documentation) but currently private-mirrors only supports contributing to the default branch of a project.

### Describe the solution you'd like

During mirror creation the private-mirrors UI should present a dropdown to allow users to select the branch that they would like to base the mirror on from among branches present on the source project. The default branch should be automatically selected to avoid adding additional selections for the most common case.

### Describe alternatives you've considered

It is possible to create a fork and change it's default branch to the desired branch so that mirrors created from that fork will use the desired branch but that requires creating a new fork for each desired base branch.

### Additional context

This issue depends on #443 to ensure that mirrors can be created based on and up to date with the latest branches present on the source project.