refactoring user.forms and user.models

1. in user.forms - use persons.models.GENDER_CHOICES - because later we will delete the GENDER_CHOICES from mks.models
2. in user.models - prevent code duplication
refactor notify application

change attribute that does a query to a property - so the query will be done only when the property is needed
change % operator to format
refactoring links app

remove use of global
change % operator to str.format
add tests
refactor apis/resources/base.py

change % operator to str.format
change dict to {}
add tests for V2 apis
Merge branch 'elazarg_refactoring' into elazarg_refactoring_2
Merge pull request #194 from OriHoch/elazarg_refactoring_2

elazarg's refactoring - slightly more complicated changes + added unit tests