Tag Management: Delete Flow

Closes #4633
- The ‘delete’ button is now a smaller plain text link, opening a
confirmation modal analog to the delete user/post flow
- Adding a post count is dependent on #4654, but the modal is already a
neat step up from the immediate, warning-less deletion.
Merge pull request #4656 from felixrieseberg/iss4633

Tag Management: Delete Flow
Navigation UI Mockup

Refs #4537

Static mockup of the navigation UI

- Includes hacky JS to simulate interactions (but not dragging, which is #4540 and no modal when deleting)
- Delete & add icons have invisible padding to increase hit area
- Drag handle has the `move` cursor
- Switches into a single column at 600px and below
Merge pull request #4665 from PaulAdamDavis/navigation-ui

Navigation UI Mockup