Add table and queries to access issued tokens data

[aslesarenko]

Explorer doesn't have proper support of tokens ATM.
However tokens belong to the platform level, thus they must be supported by Explorer.
In particular Explorer should collect a separate table of all issued tokens with all available attributes and also with probably some aggregated data (like current amount in circulation, which is updated when tokens are burnt).

[greenhat]

@aslesarenko We have an existing task for a similar purpose in the new backend - ergoplatform/explorer-backend#3 .
I've added an additional requirement to serve boxes for all issued tokens. As for extra aggregated data(amount in circulation, etc.), I'd rather keep it lean for now and add it later.