Finish up post count support in tags API

Refs #4521
- Handle 'include' query param in tags API.
- Add post_count support when fetching a tag with findOne.
- Remove post_count from options.include after processing.
- Extra database query no longer used to fetch post_count.
Add a URL preview componant

Closes #4499

- Introduces a URL preview component
- Added the component to tag settings and post settings
- Adds a new CSS file for this component which makes sure the preview never wraps onto multiple lines
Merge pull request #4686 from jaswilli/tag-api-post-count

Finish up post count support in tags API
Merge pull request #4546 from PaulAdamDavis/url-preview-comp

Add a URL preview componant
Start the Ember integration of the navigation UI