Write locale into cookies

[Robin481]

Currently if you visit the site you will be using the default_locale unless you are using the parametrized url (e.g. /fr/people).

People that changed the language probably want to keep that specific language as their default.

TODO:

• When changing the locale write it into a cookie or alternatively find better option here https://guides.rubyonrails.org/i18n.html
• Give the language in the cookie or the alternative the top priority
• Tests and stuff 🧄