Setup eslint

[GP4cK]

1. Added eslint to viewer/
2. Linted files in viewer/src/
3. Added github action to lint viewer/src

[GP4cK]

The linter fails because I didn't fix all the linting errors. You can see what these errors are if you click on Details on the right.
We have a couple of options to manage this:

1. Fix the errors by refactoring the code
2. Ignore and merge like this
3. Adjust the linting rules in viewer.eslintrc.js

Let me know what you prefer.

Note: for future PR, the linter should only run on the files modified by the PR author