Fix loading of static pages in frontend controller

fixes #1644

- Fixes bug in controller/frontend
- Created functional test for posts API to test for this bug
- Created unit tests for frontend controller
- Fixed a global variable leak in core/test/utils/fixtures/data-generator
that was leaking the DataGenerator globally
- Resolved issue that arose from fixing above bug
Merge pull request #1650 from hswolff/issue-1644

Fix loading of static pages in frontend controller
Standardize file path access throughout ghost

resolves #1390

update all string based references to file paths
to use the ./core/server/config/paths file
so that it is the single source of truth