[WIP] Collections support

[yknl]

This PR implements handling of collection scopes, collections storage bucket and encryption key generation based on the design proposal. It makes use of the identity settings feature for storage of encryption key indexes. The key indexes are used to deterministically generate encryption keys for collection buckets. (Credits to @friedger for the suggestion) Key rotation is accomplished by incrementing the index. This reduces the impact of data corruption if randomly generated keys are used instead. This is changed from the original design where encryption keys are generated based on the list of apps granted access.

Some open questions are whether the key generation logic should exist in blockstack.js or in the browser.

[friedger]

A possible use case for collection key generation in more apps (i.e. add key generation to blockstack.js) would be to collections of data that are shared with a group of people across apps.