Add Page not Found view

[gromdimon]

Is your feature request related to a problem? Please describe.
When I was navigating to the route /gene/HGNC:1000 I saw just a blank view. However, since this route is not found. We have to render 404 page.

Describe the solution you'd like
Implement 404 view. Add it to router.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
THis might be helpful: https://stackoverflow.com/questions/40193634/vue-router-redirect-on-page-not-found-404