WIP Refactor

# Goals

To separate selectors from rules. 

- CSS Selectors are defined in a `skeleton-*` file and CSS Rules are defined in a `base-*` file
- Adds a "logical" structure of the book which is the glue between the selectors and the rules

**Note**: This should **not** require regression testing because only diffs in the compiled CSS files should be comments.

A second stage of refactoring will reorganize the compiled CSS files and require regression testing.

Files that will be included in this PR:

- [x] `ccap-css3.css`
- [ ] `ccap-base.less`
- [ ] `ccap-numbering.less`

----

The "Logical" structure loosely follows:

# Page
- page number
- header (chapter title, pipe, #, section title)
- "this content is available for free ...."
- @footnotes

## Page:left/right
- corners


# Title Page
- styling for title, authors

# ToC
- Title
- Line
   - Part
   - Section

# Index
- Title
- Letter
- Entry
   - term
   - page(s)

# Attribution
- Title
- Entry
   - title, authors, url, copyright, license

# Part (Preface, Chapter, Appendix)
- title, number (or letter)
- Introduction
- Content (sections, exercises, etc)
- End-of-Chapter

## Introduction
- splash figure
- Key Concepts/Learning Objectives
- Content (sections, exercises, etc)

## End-of-Chapter
- Glossary
- Section (Summary, Homework)
- Solutions
- Content (sections, exercises, etc)

## Content

### Section
- title, number
- depth (styling based on depth)
- abstract (for sections that are modules)

### Figure
- number
- caption
- image
- subfigure

### Note
- title, number

### Example
- title, number

### Exercise
- title, number
- Problem
- Solution

#### Problem
- title
- Content (sections, examples, etc)

#### Solution
- Content (sections, examples, etc)


### Equation
- title, number

### Table
- title, number
- caption
- header
- row/cell

### Footnote

### Link
- external
- target-type
- number lookup