Handle conflicting and duplicate page break rules on the same element

[evnbr]

Bindery breaks if multiple instance of FullBleedPage and PageBreak are applied to the same element. There are 3 stages to fixing this:

• [X] 1. Make sure only one instance of each rule is applied to an element
• [ ] 2. Use CSS selector specificity to determine which rules take precedence
• [ ] 3. Merge properties where appropriate.

[evnbr]

better short-term strategy here is probably just throw a conflicting rule error