Rework partner organizations list and optimize logos

[utarwyn]

I'm currently working on improving the site's overall ecological footprint. This PR is one step further by proposing the following changes:

• Optimize logos of all partner organizations to suit the size displayed in the browser
  Docs: https://developer.chrome.com/docs/lighthouse/performance/uses-responsive-images/
• Use webp or svg images to decrease the bundle size
  https://developers.google.com/speed/webp
• Simplify the definition of companies in the json file
• Create a common component between the home page and the dedicated page to factorize code
• Create a css variable file to easily find all the colors used (in the future)

For developments that impact a bit more code, I'll go through PRs like this one. For small subjects, I push them directly onto the main branch. I hope that's ok for you.