Add LastLoginTimestamp to User Model

[raclim]

Changes:

• Adds lastLoginTimestamp to the User model, which is updated every time a user logins to their account
• Sets express session cookie expiration period to 4 weeks. Before it was "indefinite" by default since there was no maxAge or expires set.
• This is being added to help determine inactive accounts.

I have verified that this pull request:

• has no linting errors (npm run lint)
• has no test errors (npm run test)
• is from a uniquely-named feature branch and is up to date with the develop branch.
• is descriptively named and links to an issue number, i.e. Fixes #123

[release-com]

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