Consider using `FactoryGirl.build_stubbed` instead of `FactoryGirl.create` since it's way faster

FactoryGirl provides a build_stubbed method that stub all the object attributes.

I have the feeling that we could speed up a lot of our unit tests quite easily with a simple change:

                                   user     system      total        real
build_stubbed(:abuse_report)   1.380000   0.130000   1.510000 (  1.530494)
build(:abuse_report)           6.740000   0.330000   7.070000 (  7.795627)
create(:abuse_report)          7.010000   0.290000   7.300000 (  8.117727)

                                   user     system      total        real
build_stubbed(:user)           0.740000   0.100000   0.840000 (  0.860718)
build(:user)                   0.610000   0.030000   0.640000 (  0.651007)
create(:user)                  3.350000   0.180000   3.530000 (  3.896695)

                                   user     system      total        real
build_stubbed(:project)        1.940000   0.170000   2.110000 (  2.141562)
build(:project)                8.030000   0.360000   8.390000 (  9.250054)
create(:project)              13.270000   0.580000  13.850000 ( 15.675928)

Interesting thing to note: using build is still better than create (but associations are still persisted!) but build_stubbed is a deal-breaker here since even associations are stubbed!

Of course, that won't work for feature specs where we need objets in DB, obviously!