Non-micro optimization for the main page - cache projects

At dev.translatewiki.net this saves 150 ms from the median
page serving time:

Before:
              min  mean[+/-sd] median   max
Connect:        5    6   1.2      6       9
Processing:   377  404  23.3    400     509
Waiting:      377  404  23.4    400     509
Total:        382  410  23.4    406     514

After:
              min  mean[+/-sd] median   max
Connect:        5    6   0.9      6       8
Processing:   216  248  22.4    241     306
Waiting:      216  248  22.4    241     306
Total:        223  254  22.5    249     311

Change-Id: I3e73315241bc7f060fe27df9b4a6c0e06b1b2069
Merge "Non-micro optimization for the main page - cache projects"