Move "api.js" into its own place

[dpordomingo]

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from https://github.com/src-d/gitbase-playground/pull/16#discussion_r187622649

We could move frontend/src/api.js into its own place as done in CAT for example.

I see two reasons:

• first one is kindof opinionated: imo the root dir should be populated with as fewer things as possible because it helps to understand better the structure of the code.
• second one is generic: now we have in this file the logic (copy-pasted from cat) to connect with the API (business agnostic), and also the "repository-like" code (business related). Splitting both parts properly, and moving business agnostic to a service, and the business logic to a repository would help us to reuse and test better.