Page Builder v0.5.5 - Imported Sites and Migrations

Fixes for imported header and footer chrome, page settings that went missing during a site migration, and a licence field that rejected valid keys.

Fixes

• Imported chrome sets its own design mode - importing a header and footer now switches the site to Imported mode automatically. Sites that pasted their own chrome were left in Framework mode, so the preset stylesheet and brand fonts loaded underneath the imported CSS and fought it. Choosing a mode by hand still wins.
• Page settings survive a migration - page settings, per-page CSS and per-page JS can be read and written over the REST API. They used to be dropped silently, so a migrated page arrived unstyled and then had its markup reflowed the first time anyone saved it.
• Licence key field - the typing hint compared against the wrong length and asked for "2 more characters" on a perfectly valid key. The key itself always worked.
• Rolls up assorted beta fixes accumulated since 0.5.4.