UI to reassure users that adding the first dataset is working

Four things made me nervous as a new user uploading a file for the first time:
1. When adding what felt like a small test dataset ([Cars: 5000 x 21](https://corgis-edu.github.io/corgis/csv/cars/)), the 'ingestion' took several minutes on my laptop. Using `htop` I realized that it was doing a lot more work than I was expecting, which is fine but the UX of that spinner was confusing and off-putting for my first attempt at using the software. Maybe a progress bar or light log output. Even just a total time estimate with the spinner would be OK
2. My web UI reset and the screen returned to an empty DataSets screen with just an Add New button. Using htop, I could tell that it was still working, and I was able to successfully refresh the screen after my laptop fan stopped whirring, but again it was off-putting as a new user to feel like it failed after I'd been waiting
3. It would be nice to have the "Upload a csv..." blue box disappear once I'm on the second step (feature verification) because I don't imagine there's a use case for uploading a second file immediately, and it's a little confusing (again, trying to make this a no-brainer for new users)
4. I tried to set Year as an ordinal, but it gave me an error and I had to re-upload. Leaving it as a numeric worked. I accidentally left the mode to 'classification' with mpg as the target, and got a nicely explicit but avoidable error ("Unable to upload file. Error: Datafile validation failed, Classification datasets must have at least 2 rows per class, class(es) '[35, 36, 38]' have only 1 row.") After about 3 minutes of the spinner, I got a box "Error: Something went wrong". Going to the datasets page, I don't see the cars data, but `htop` tells me that it's still working in the background, and having tried this a few times I actually think it's going to appear when I refresh the page after get_metafeatures.py finishes (EDIT: It looks like it didn't, actually). Again, any user not comfortable going into htop might have given up if this was their first upload attempt

I want to underscore that I'm really excited about the vision of this software and want it to succeed! I've long been a big fan (and one-PR contributor, FWIW!) of TPOT and I think these tools are so valuable when freely shared