Display only the number of public competitions in the highlights

[Didayolo]

Here is how the statistics look like in the home page:

I think we should display only the public competitions. Another possibility is to at least remove the redundant "private competitions" information. Or even keep only "total competitions" because other statistics are total too. That is just too many numbers for unrelevant information (especially since most private competitions are just there for testing purpose).

Same the goes with CodaLab highlights page:

[Didayolo]

https://github.com/codalab/codabench/blob/develop/src/apps/pages/views.py

https://github.com/codalab/codabench/blob/develop/src/templates/pages/home.html

[ihsaan-ullah]

Changed in branch private_competitions