Validate that a project exists in public GitHub before it can be created in GitProxy

**Is your feature request related to a problem? Please describe.**
Currently, when creating a new project within GitProxy, admin users are able to enter any details regardless of if the project exists in public GitHub.

**Describe the solution you'd like**
When attempting to create a project in GitProxy, the public GitHub API should first be queried to ensure the project exists:
- Organization
- Repository name
- URL