fix #26 allow adding members that share names with deleted members

[noobte]

Description

Made the input validator check only for existing users Refactored the delete member function, creating a private method for scheduling the deletion. Added a parameter for updating the name. Made the delete member function call the schedule function with the updated name.

Deleted users are renamed to "[name] (deleted: YYYY-MM-DD)"

Screenshots

Bob deleted: New bob joins the team:

Quality checklist

• [x] Self review: I have reviewed this PR myself before opening it for review.
• [X] Tests I have done:
• tested deleting members and re-adding them
• tested adding duplicate members (doesn't work which is correct)
• tested renaming new bobs after previous bob was deleted.