Rename invoice_condition_template and improve its UI

Rename invoice_condition_template to invoice_comment_template, and base_condition_template to base_comment_template (as proposed here: https://github.com/OCA/sale-reporting/pull/18#issuecomment-64691382)

Move the condition fields in a new page

They take too much place on the 'Order Lines' page. Also I improved the
general layout of the fields.

Rename 'Condition' to 'Comment' because the texts are not 'conditions'

Same changes have been done in sale_condition_template: https://github.com/OCA/sale-reporting/pull/18