Fix 404 returned as JSON instead of HTML.

[shrihari-prakash]

Describe the bug
At the moment, all 404 errors are handled by express, and this results in a JSON response being returned when a non-existing page from static is called. This is not very elegant and ideally this needs to be handled by React. However, this means, the server will send a 200 even for non-existing route even though it is a 404 and only then the frontend will know about the non-existing page. This situation needs to be fixed to wither send index.html with a 404 in invalid route or have a separate API prefix and handle frontend and backend 404s separately as @juliolopeztorres suggests.

To Reproduce
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2. Click on '....'
3. Scroll down to '....'
4. See error

Expected behavior
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Desktop (please complete the following information):

• OS: [e.g. iOS]
• Browser [e.g. chrome, safari]
• Version [e.g. 22]

Smartphone (please complete the following information):

• Device: [e.g. iPhone6]
• OS: [e.g. iOS8.1]
• Browser [e.g. stock browser, safari]
• Version [e.g. 22]

Additional context
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