Augment existing data model w/ learnings from github.com/community-graph

[halcyondude]

• github.com/community-graph
• github.com/community-graph/documentation

Here's the model we have today:

...and how it relates to the community-graph's model:

Here's the community-graph model:

Note that the community graph data import uses the GH GraphQL api. In this project (landscape-graph) the bulk of git info will be coming from src-d/gitbase, however having the interactive / GraphQL mechanism is also useful.

• The git model is already quite close, however the current landscape-graph model doesn't model Issues, and it probably should.

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• Add Issue to the Git Model
• Determine what else we should take (now), and what to do in the future (new issues)