Normalize redux data and introduce backlinks.

[oliverbutler]

Look into https://github.com/paularmstrong/normalizr

Currently

Redux stores pages as an array of blocks with no children
To query a pages children we currently just filter all pages, this has some benefits

• Small number of pages not much slowdown
• Easy to update records this way

Proposed

Work out how best to store, in a normalized way:

• Outline of all pages (for sidebar etc)
• For certain blocks, all of their content/children
• Database/property content

Ideally - we want no duplication of data.

[oliverbutler]

Using a normalized tree structure in redux would also enable ordering of children, a current flaw with the existing implementation.

This is a high priority fix.

[oliverbutler]

• Update SQL to support tree ordering
• Update redux store
• Update sidebar (may be able to remove tree-building)