refactor(graphql): give each cell an id and a summary object

[CammilleCC]

This PR gives some structure to the cell: it now as an ID (in the form of {database}:{proposal}:{run}:{name}) and a summary field. This helps Apollo caching the cell by ID, regardless on how many times we write it, specifically:

• when a page loads
• when the heavy values arrive
• when there's live update

The new schema is as follows:

    type Cell {
      id: ID! 
      name: String!
      error: CellError
      summary: CellSummary!
    }

    type CellSummary {
      value: Any
      dtype: DamnitType!
    }