Allow user to set static page before saving

fixes #1350
- jQuery event.preventDefault() stops the user from selecting the checkbox
Pass proxy Ghost interface to Apps

Closes #1478

- Create new proxy.js that exposes createProxy method
- Pass proxy to App activate/install in lieu of Ghost instance
Fix wrong error message

closes #1466
- added status code for error object
- added test for frontend errors
Lowercase email address.

fixes #1498
Add schema.js

closes #1398
closes #1399
closes #1400
- added schema.js with database version '000'
- refactored migration to use schema.js
- if new table is added to schema.js and databaseVersion is increased, table will be added
- if new table is deleted to schema.js and databaseVersion is increased, table will be deleted
- alter table from issue #1400 is delayed until knex supports column modification
- changed import pre checks to work again (will be refactored separately)
- added basic PostgreSQL support (Attention: not supported/tested)
- changed error handling in server.js
Merge pull request #1489 from sebgie/issue#1466

Fix wrong error message
Merge pull request #1505 from halfdan/1498-lower-email

Lowercase email address.
Merge pull request #1353 from thgaskell/1350-static-page-checkbox

Allow user to set static page before saving
Correct the Markdown for emphasis
Merge pull request #1485 from Archanium/MinorIssues

Danish language file and a correction to Markdown modal
Merge pull request #1449 from sebgie/issue#1398

Add schema.js
Merge pull request #1482 from jgable/appProxy

Pass proxy Ghost interface to Apps