Git Branch Checkout & Redux Thunk

[nelsonni]

Git functionality to allow per-card git branch switching so that multiple versions of a single file can simultaneously exist on the canvas. This addresses the final feature open requirement in #68.

Includes:

• Adds Redux Thunk dependency to provide Redux middleware for writing sync and async logic that interacts with the Redux store
• Adds beta versions of Thunk Action Creators for React container functions that require synchronicity with the Redux store
• git2.getRepository() simplifies, and will replace, git.extractRepo() (currently beta)
• metafile2.getMetafile() simplifies, and will replace, metafile.extractMetafile() (currently beta)
• handlers.loadCard() provides additional safeguards against missing filetype handlers and extends functionality to accept either a Metafile or a filepath (currently beta)
• New React components: ErrorDialog
• TypeScript data structure utility functions (e.g. deserialize(), removeUndefined()) moved to formats.ts

[jettseale]

The dispatch calls in Browser.tsx can be simplified down now that Redux Thunks have been integrated. Besides that, just some minor changes to improve readability.