Make database setup and teardown idempotent

[angela-tran]

Currently, an attempt to restart a server Docker container will result in an IntegrityError:

+ python setup.py
Creating table...
Table created.
Importing users from /.devcontainer/server/data.csv
Traceback (most recent call last):
  File "/usr/local/lib/python3.9/site-packages/sqlalchemy/engine/base.py", line 1819, in _execute_context
    self.dialect.do_execute(
  File "/usr/local/lib/python3.9/site-packages/sqlalchemy/engine/default.py", line 732, in do_execute
    cursor.execute(statement, parameters)
sqlite3.IntegrityError: UNIQUE constraint failed: user.key

A separate but similar issue is that an attempt to teardown the database if it has already been torn down results in an OperationalError:

+ python teardown.py
Traceback (most recent call last):
  File "/usr/local/lib/python3.9/site-packages/sqlalchemy/engine/base.py", line 1819, in _execute_context
    self.dialect.do_execute(
  File "/usr/local/lib/python3.9/site-packages/sqlalchemy/engine/default.py", line 732, in do_execute
    cursor.execute(statement, parameters)
sqlite3.OperationalError: no such table: user

We should make the setup and teardown scripts idempotent to prevent these errors.

Additional context

#101 made it so the app container initializes the database on startup.