Disabled the add/remove button as appropriate.

The add button is now disabled if there are no more requirements to select from.

The remove button is now disabled if there are no more requirements to remove from the game (such as at the start of creating a planning poker session.
Add the database functionality for votes.

authors: rbkillea
Merge pull request #119 from hlong290494/sbl-dev

Moderator field for the PlanningPokerGame model
Merge pull request #118 from sfmailand/sbl-dev

Disabled the add/remove button as appropriate.
Merge pull request #120 from ryanbaylorkillea/sbl-dev

Add the database functionality for votes.
Adds an export button that outputs selected requirements to a file.
This file is created if it is non-existant.
This file is currently in wpi-suite/Janeway and named requirements.txt.

TODO: Allow the user to input their own file name
TODO: Move the button to a better place
TODO: Add transaction history and acceptance tests to the file output

Manually merge by Long Nguyen Duc Hoang <lhnguyenduc@wpi.edu>
Original codes by benkorza.