Add initial code for TOTP support

- Change LoginHandler to so that it accepts and checks TOTPs from client
- Add redundancy for unsynchronised clocks between client and server (when one's clock is behind/ahead of the other's)
- Change RegisterHandler such that it adds the same TOTP seed to each registered account (For testing only; to be removed later)
Tweak main page layout

- Change input fields to support the placeholder attribute
- Shrink input window width
- Adjust positioning and size of input fields
- Tweak jQuery code for animating buttons to support new input window dimensions