Release Planner app: Check GitHub user permissions before accessing SDK release app

New feature request from @catalinaperalta

**Scenario:** Ensure that user has the appropriate permissions as they start to work on creating and shipping SDKs.

Questions:

- [x] When should we validate and check for GitHub permissions?
- [La Donna] To access the API readiness and SDK release apps user must be signed into GitHub. So, permissions need to be validated in the Release Planner UI flow.
- [x] What level of permissions is needed to do what activity in GitHub? Create a PR? SDK release?

**UI Requirement (new UI):**

- [x] UI requirement: Include a step in current UI flow or mandatory section in the app where the person can input the github alias of the person who will be checking in to the relevant repositories.

**Backend Requirement:**
- [ ] This new step could ping some endpoint (something on APIview or one of the Github repositories, such as azure-rest-api-specs) that would verify that they have the correct permissions to begin work.
- [ ] If they don’t have access, could the permissions request be sent and/or granted through the app? At least, it would be good if it blocks or sends some reminder to get the correct permissions. [Catlina] I think this would help service teams who are just coming to us for the first time to get started in a better way and they’ll be less likely to get lost in how to resolve these permissions issues.