Add eslint to enforce coding conventions

[lauramosher]

Why

Coding conventions are documented best by a linter that can callout discrepancies early and often. In other Ministry Centered projects, eslint is the linter of choice. This sets the standard of the code. It removes any question of what is acceptable and what isn't. It also means that once we get to reviewing code to add to this project, we can spend more time reviewing the feature or patch rather than picking at coding conventions and style choices.