Further implementation of database

1. login/sign-up page, with check on login error, existing user, etc.
2. Add page: should be interfaced with QR scanner. Right now, assume a
scanned userID can be input to the bottom box. Then the info of scanned
user will be displayed on the page. If press "add" button, the userID
will be added to the collection of the current user.
3. testDB page: used to illustrate the collected userIDs. This page
should be replaced with the Explore page, so that each collected userID
is used to retrieve corresponding info from the web and displayed on
Explore page
4. Log out page