[Backend] #128, implemented HARD DELETION of activities and users.

1. When a user is deleted, all the user's activities will be deleted.
2. When an activitiy is deleted, all the people joining that activity will automatically unjoin and all the comments associated with that activitiy are all deleted as well.
[Backend] #174 now works temporary with the transformer format
local works
[Backend] gonna delete .idea
[Backend] delted vendor and .idea file
[Backend] changed gitignore .idea and vendor