{{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)
Remove unused, commented out CSS.

See #4102
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
Merge pull request #4129 from halfdan/fix-leave-modal

Fix leave modal when leaving new post
Incrementing hard limits for Meta title and Meta description

closes #4124
- incremented values for client-side validation for Meta title and Meta description
- changed error messages to display the maximum number of characters.
Fix post settings menu cog alignment and colours
SERP style updates
Fix padding on content preview mobile views
Minor start on CSS Style cleanup

See #4107