[READY] Login System

**Scenarios:**
- [x] User logs in with nothing -> "Please enter your username and password."
- [x] User logs in with either username or password missing -> "Please enter your username and password."
- [x] User logs in with wrong user/password -> "Wrong username and/or password"
- [x] User logs in with a non-existant user -> "Wrong username and/or password"
- [x] User navigates to /mycell when they are not logged in -> redirect to /login & "You must be logged in to view this page."
- [x] ~~User navigates to /login when they are logged in -> redirect to /mycell~~
- [x] ~~Logout button only appears when logged in~~
- [x] ~~User clicks logout on any page -> redirect back to that page~~

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