Note this commit also introduce a new {{d-popover}} component, example usage:
```hbs
{{#d-popover |state|}}
{{d-button label="foo.things" class="d-popover-trigger"}}
<div class="d-popover-content">
Some content
<div>
{{/d-popover}}
```
Adds a custom bookmark-clock icon to discourse-additional.svg for use with the new bookmarks with reminder functionality.
Also add some code to correctly refresh the post-stream icon for bookmark to show the clock after save.