fix: cast table max upload size to int

[rchandnaeg]

Environment variables are loaded in as strings, which results in the following error for loading QuerybookSettings.TABLE_MAX_UPLOAD_SIZE

querybook_web            | Traceback (most recent call last):
querybook_web            |   File "/opt/querybook/querybook/server/app/datasource.py", line 84, in handler
querybook_web            |     results = fn(**kwargs)
querybook_web            |   File "/opt/querybook/querybook/server/datasources/event_log.py", line 18, in log_frontend_event
querybook_web            |     events = json.loads(flask.request.data)

This fix casts the table max upload size variable to an int before applying it to the flask app config

[czgu]

thank you!