Generate more realistic tree shapes in sample data

[chillu]

• FTPageMakerTask only creates 2k pages, yet our internal NFRs dictate 10k pages.
• It only has 100 pages under one node tops, but should have a random amount between 10 and 1000 (above the default node_threshold).

[sminnee]

Another important consideration is to have a significant number (50%?) draft pages. A few performance issues specific to draft pages have slipped through the cracks recently.

[sminnee]

Some examples:

• FIX: Add _complete flag to ensure that missing record requests are ca… tractorcow-farm/silverstripe-fluent#475
• FIX: Performance optimisation for draft pages in treeview silverstripe-versioned#181