[DashAuth] Specify cookie `max_age` to prevent immediate expiry

[davidhpark]

Currently, the max_age parameter is not set, resulting in the immediate expiry of the cookies. This forces users to always log in whenever they close their browsers. One solution would be to set the max_age to a week in SolveBioAuth.

Related: solvebio/www.solvebio.com#1580

[davecap]

Ideally we can make this a configurable parameter somehow, possibly when creating the Dash app object. One way to do this is to add the ability to pass a custom SolveAuth object through __init__.