Add "empty" content and tooltips to Dashboard

[am9u]

These relatively minor copy / UI updates help to explain the Dapp and the Dashboard to the user.

https://consensys.invisionapp.com/share/PBO9TOG4M39#/321307947_Dashboard_-_Empty_State

https://consensys.invisionapp.com/share/XCQZESGTVJH#/346462155_Dashboard_-_Empty_State_-_Challenges

https://consensys.invisionapp.com/share/5QQHQ3KC9E3#/321397401_Dashboard_-_Empty_State_-_Applications

https://consensys.invisionapp.com/share/8HOAATZZJDN#/322343533_Dashboard_Tooltip_Rewards

https://consensys.invisionapp.com/share/65QZETZA4BR#/322343534_Dashboard_Tooltip_Voting_Balance

https://consensys.invisionapp.com/share/ZKQZEUFYPED#/322343535_Dashboard_Tooltip_Available_Balance