Enhance/rest nextjs api routes

[mayank1513]

Fixes #4974

Refractor code to match community standards

• [x] Remove unnecessary React.FC syntax
• [x] Extract Layout to _app.tsx (No need to add layout on every page)
• [x] Use absolute path uniformly - remove import ../.., etc.

Update UI to reflect functionality properly

• [x] SignUp button should be renamed to "Add Author"
• [x] /signup route should be renamed to /add-author
• [x] On create post page, email input should be changed to select (As only those emails that exists in users table can be added there)
• [x] Remove legacyBehaviour and update links