Introduce eslint-config-galex

Here it is! I brought our overrides from H5Web as a base. Then, to ease migration, I chose to disable most of the failing rules via .eslintrc.js. My goal is to try to re-enable those rules and fix them in the codebase a few at a time. I think it will be easier this way to decide whether each rule applies to daiquiri's coding style, is worth the effort of fixing, is worth fixing in TS files but not JS files, etc.

For the time being, with this PR, the important is:

• to make npm run eslint pass in the CI so we can remove allow_failure: true (ongoing work);
• to lint JSX files, which were not being linted until now;
• to introduce a stricter, less opinionated config that covers more ground (notably TypeScript) without having to configure any plugins/overrides ourselves.

The diff is a bit long because I did end up auto-fixing a few rules that, I think, are very easy picks, like obsolete eslint-disable directives and React fragments.