[FIX] doc website: application field does not exists on ir.ui.view. The optional inherit used customize_show and active fields
Merge pull request #3235 from odoo-dev/8.0-doc-website-template-optional-chm

[FIX] doc website: application field does not exists on ir.ui.view.
[IMP] fields: reuse column objects when possible, instead of recreating them

This is a memory optimization: it reduces the memory footprint of each
registry.  We have observed a reduction of 10Mb on a database with modules crm,
sale, purchase, stock.