Summary:
- Include ES6 files in spec-suite
- Fix template store specs
- Refactors TemplateStore a little bit to adjust to specs
- Convert coffe .cjsx files to ES6 .jsx files
- Fix TemplateDraftStoreExtension functions
- Update ComposerTemplates example README
Test Plan: - Plugin unit tests
Reviewers: bengotow, evan
Reviewed By: evan
Differential Revision: https://phab.nylas.com/D2250
