Display a message to the user if visualization request takes long

**Is your feature request related to a problem? Please describe.**
If a user fires off a computation-heavy visualization request, they will feel forced to patiently wait for the request to complete.

**Describe the solution you'd like**
If the user navigates away from the current visualization type, display a popup:
"Cancel computing this visualization?"
- with choices "yes", "no (your visualization will be available from the download menu)"

This popup would only be displayed for logged in users, for non-authenticated users, the visualization tasks would always be revoked.

After a number of polls, display a popup to the user:
"This request will take a while. Cancel computing this visualization?"
With same choices as above. Again, only logged in users would get to see this message: non-authenticated users will have to wait.