Standardise field order

[stephen-cox]

Spun out of https://github.com/localgovdrupal/localgov/issues/487

For step-by-step:

• the "step by step overview" field should be first and a lookup type, not drop down menu
• the field should be labelled "parent page"
• it's type should be a lookup not a drop down menu

[willguv]

@stephen-cox thanks for breaking these out