Add a field to the DB entry for each DApp to show staked amount

[andytudhope]

It would be useful if we could tell, just by looking at the DB, which DApps have staked SNT, and which have not, as this allows us to tell which are stored in the contract, and which are just in the DB (which is useful for administration purposes).

Should be a fairly simple thing to add here.

[andytudhope]

Ironically, our indexedDB actually already has a field for this called sntValue, so it's just a question of making sure mongoDB also does.