Replace nodejs-bcrypt with bcryptjs

* https://github.com/shaneGirish/bcrypt-nodejs
* https://github.com/dcodeIO/bcrypt.js
Merge pull request #1248 from halfdan/bcryptjs

Replace nodejs-bcrypt with bcryptjs
Add validation for importer

closes #952
- moved api.js to api/index.js
- added api/db.js for import and export functions
- moved /ghost/debug/db/export to GET /api/v0.1/db
- moved /ghost/debug/db/import to POST /api/v0.1/db
- removed /ghost/debug/db/reset
- added validation for import
- added constraints object to migration
Merge pull request #986 from sebgie/issue#952
Fixed style of modal in preview.

fixes #1179
Merge pull request #1202 from halfdan/1179-modal-styles
PaaS