Feature/show 404 page when URL doesn't exist

[roienatan]

Currently if one tries to load a common with an id that doesn't exist, an infinite loader is appearing. That's a bad UX and we should show a generic 404 page instead.

A use case that a user can encounter this is after deleting a common for example.

[roienatan]

@HannaMaklakova I think it's good to have an official design from brio regarding that, but for now we can show basic 404 page as it's important.

[roienatan]

New design and a PR #310