Add Gmail-style shortcuts

Ref #3940
- add `j`/`k` for navigating posts down / up
- `c` for opening a new post
-  `o` to open the selected post
- update keymaster dep to allow for multiple key unbinds (`key.unbind('up, j')`)
Fix minor IE UI issues

Closes #4088

- Fix Flexbox issue with the user list
- Fix positioning issue with the mobile menu toggle icon
- Make the mobile settings nav use absolute positioning (as fixed elements don't move inside a `transform: translate(...);` parent
{{gh-path}} Handlebars helper, returning paths

- Uses utils/ghost-paths to resolve paths
- {{gh-path}} or {{gh-path ‘blog’}} for Ghost’s root (mysubdir/)
- {{gh-path ‘admin’}} for Ghost’s admin root (/mysubdir/ghost)
- {{gh-path ‘api’}} for Ghost’s admin root (/mysubdir/ghost/api/v0.1)
Merge pull request #4090 from PaulAdamDavis/ie-fixes

IE UI fixes
Remove unused, commented out CSS.

See #4102
More robust deprecation check code

Closes #4082
* reformatted code to allow for traversal
* deeper config items should be denoted like this: `object.object.object.property`.
* added tests for testing the deprecation warnings
Deprecated mail.fromaddress, mail.from is Title <email@address>

Closes #4018

* cleaned up `mail_spec.js`
* deprecated `mail.fromaddress`
* implemented 'Blog title <email@address.com>' format with fallbacks
* added tests to deprecation and from address, made existing ones more robust
* moved domain intuit into its own module: `GhostMailer.getDomain()`
Merge pull request #4085 from javorszky/iss4018

Mail: deprecated mail.fromaddress, reformatted mail.from to Title <emailaddress>
Implements is-helper

closes #2249
- Added context to res.locals
- Added context aware is helper
- Added unit tests
Fix chevron animation in publish button

Closes #4104

- Fixed the broken spiny animation used in the publish button
Update meta_* helpers.

no ref
- Return meta_description from post.meta_description if set
- Return meta_title either from post.title or post.meta_title if set
- Adds tests
Merge pull request #4093 from halfdan/update-helpers

Update meta_* helpers.
Merge pull request #3962 from halfdan/2249-is-context

Implements is-helper
Merge pull request #4117 from PaulAdamDavis/fix-post-state-arrow

Fix chevron animation in publish button
Updating messages to use errors module

no issue

- only updated the messages that are errors or warnings
- changed the structure of the messages to follow the 3-arg format
Remove placeholders and limit SERP description

closes #4112, closes #4113
- Remove meta_* placeholders
- Add &hellip; in SERP preview when desc too long
Merge pull request #4115 from halfdan/4113-serp-placeholder

Remove placeholders and limit SERP description
Merge pull request #4127 from ErisDS/msg-update

Updating messages to use errors module
Fix leave modal when leaving new post

no ref
- set titleScratch to empty string if unset
Fix GhostTab and GhostTabPane array dependencies

No issue
- TabPanes weren't finding their tab due to not listing all their dependencies in Ember.computed