Fixed check for a theme's custom error.hbs:

Closes #2513
- Checks for property `error.hbs` on active theme
- Added unit test to ensure `error` view is rendered when activeTheme has
  a custom error template.
- Removed unused variable, `userErrorTemplatePath` from errorHandler
- Refactored errorHandler.`updateActiveTheme` to take one argument, the new active theme, and to then check if the active theme has an error.hbs
- Changed errorHandler unit test to use rewire for mocking config.
Ghost footer outputs minified jquery in production

Closes #2524

- Added minifying jquery in grunt prod task

- Add test coverage for altered jquery
Remove second PUT request on image save

closes #2557
- replaced model.save() with model.set()
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Merge pull request #2517 from novaugust/theme_error

Fixed check for a theme's custom error.hbs
Merge pull request #2558 from sebgie/build-fix

Remove second PUT request on image save
Merge pull request #2545 from joeljfischer/minify-jquery

[WIP] Ghost footer outputs minified jquery
Merge pull request #2561 from t3chnoboy/master

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