Sign and add refactor

[CharlieHewitt]

Refactor done as part of #5

Summary of changes:

• created add_signup() to add a user to their role in an operation (no validation on role being full -> check before calling)
• refactored -sign and -add commands to use add_to_operation() for their common logic (validation, add_signup, write to json, update pinned post)
• added missing method description comment to remove_signup()

Potential improvements for the future:

• finish Automate role management #5
• Deal with the 'error cases' in add_to_operation() in a different way (sending error messages ...) -> try not to pass context into add_to_operation() so it becomes independent of discord.py -> could be useful in the future.