[Spike] Migrate issue boards from Vuex+Apollo to Apollo only

Currently, our boards implementation uses Vuex for state management and Apollo for the transport layer. This makes an application heavy (as we have more dependencies), the logic bloated (we need to do additional work for loading, error handling, and data updates), and the synchronization with the sidebar much harder (we need an events system to synchronize Vuex state of the boards with Apollo state of the sidebar). It also makes real-time boards harder to implement as we'll need to update the Vuex state manually on every subscription trigger.

Proposal

Instead of having two sources of truth, we should remove Vuex altogether and rely on Apollo cache exclusively to store boards data.