Improve membership endpoint

Currently setting expiration date is the responsibility of the FE. The FE doesn’t know what time to set to the expiration date, and it cannot also be left empty as it is a required field. Let’s make the field optional to give the FE to set, if they set it to empty we will have to add the value from environmental variable.

Additional fix: On space ms when returning invited members we should also attach the invitation id along with them. This helps the FE to send delete invitation request using the given id.