Including theme partials in a way that supports symbolically linked directories

closes #1937
 - using fs.stat() instead of hasOwnProperty() to test for directory existence
Fix for session handling

closes #1978
- wait for async calls before callback()
Adding guard to meta_title

fixes #1992
Merge pull request #1949 from jolros/symlinked-themes-bugfix

Including theme partials in a way that supports symbolically linked directories
Merge pull request #2004 from ErisDS/issue-1992

Adding guard to meta_title
Upgrade express-hbs, and switch to using registerPartial

fixes #1964, fixes #1975

 - Issues with partial handling which caused #1964  have been part fixed by handlebars, part worked around by express-hbs, we must use `registerPartials` to ensure partials are handled correctly.
 - Issue with error handling which caused #1975 has also been fixed in express-hbs, which now catches the error from handlebars and passes it to express so that we can handle the error with an error page.
Use absolute path when storing images

closes #1983
Merge pull request #2017 from ErisDS/issue-1975

Upgrade express-hbs, and switch to using registerPartial
Merge pull request #1979 from sebgie/fix-session-1

Fix for session handling
Start up safely when the activeTheme is not present

fixes #2000

- resolves errors when attempting to start Ghost without the active theme present
- the frontend will render a 500 error page safely
- issues with themes that have an error template are resolved separately in #2018
Catch errors when rendering the error template

fixes #1991

- if there's an error when rendering the error template, default to sending a plain response detailing both errors.
Merge pull request #2018 from ErisDS/issue-1991

Catch errors when rendering the error template
Merge pull request #2025 from ErisDS/issue-2000

Start up safely when the activeTheme is not present
Have NProgress called for all ajax calls

fixes #2011

- rather than trigger NProgress for fetch calls
 I’ve moved it to be used for all sync calls

- issue #2011 was a result of NProgress only being
 called during a fetch() call, which when saving
 is not used - save() is.  Sync is used by all
 ajax calls.
Merge pull request #2032 from hswolff/fix-2011

Have NProgress called for all ajax calls
Merge pull request #2024 from jgillich/0.4-maintenance

Use absolute path when storing images
Improvement for published posts with no published_at

issue #2015

- this is another little workaround / improvement to try to reduce the number of people who end up with a published post with no published_at set
- I assume we need to complete #1655 to fix this properly
Merge pull request #2038 from ErisDS/issue-2015

Improvement for published posts with no published_at