add page not found view

[plebeius-eth]

Routes are in src/app.tsx, view components are in src/views.

This issue is about making a new folder in src/views, /not-found/, containing index.ts, not-found.tsx and not-found.module.css (like all other components).

The not found page should probably look similar to the ones on old reddit, but we shouldn't use reddit's mascot at all.

If the user types any route that is not a view component, it should redirect to this not found page. I think at the center of the page there should be a link to the homepage.

[plebeius-eth]

user should be redirected to not found page immediately if a cid is not exactly 46 characters, not alphanumeric etc. Should also check the subplebbit address (if does contain '.' or not, etc). This is to prevent useless loading times for invalid links

[plebeius-eth]

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