SiteServiceImplTest: reduce test cases duplication by adding tests for logEvent()

Currently all methods in `SiteServiceImpl` invokes `logEvent()` under the hood. We have similar test cases for them, but they are mostly testing `logEvent()`. I suggest make `logEvent()` visible for testing and add tests for it. Tests for `logAbout*()` method should only checks that the method invokes `logEvent()` and passes right arguments to it.
