users management

**Ready to merge**
Fixes #28

- [x] Include yourself (current user) in the users table (cannot remove yourself though)
- [x] Remove real-time email validation for the “add user” modal. Show an error message instead (on submit) if “@“ is omitted.
- [x] Order users table by most recently added to earliest
- [ ] Display date the user was added to the marketplace, not the creation date of the account (assuming the email is already in use)
- [x] Rearrange the column order of the API Keys and Users table:

The 4th item requires a change in injustice.