Refactor, phlcon_user: add_hint* -> add_user_hint*

Make the intent of these functions clearer by using 'username' instead
of 'user'.

Test Plan:
Make substitutions using rope-rename, manually check diff for unexpected
changes.

Look for missed substitutions
$ git grep add_hint

Update doc, standard tests
$ ./precommit.sh
Refactor, UserPhidCache->UsernamePhidCache

Make the intent of this class clearer by using 'username' instead of
'user'.

Update the names of local variables in the class' methods.

Test Plan:
Make substitutions using rope-rename, manually check diff for unexpected
changes.

Look for missed substitutions of the class' name
$ git grep UserPhidCache

Update doc, standard tests
$ ./precommit.sh