Refactor Mutation for me path

[brettski]

Task is to refactor the mutation to add a me path which will provide users with their CRUD of sessions.

Create a new me path returning type MeSessionsMutation. Mutations:

• create
• update
• withdraw
• propose

This allows create for things like THAT.us and open spaces sessions, and propose for CFP needs.

• The current methods used for creating will be marked as deprecated (via directive)

There is already an admin path in sessions for admin level edits via GraphQL