Harmonize timestamp naming

[wvengen]

While Rails has settled on created_at and updated_at, we have created_on et al. Let's bring some harmony here.

• Deliveries#delivered_on
• FinancialTransactions#created_on
• GroupOrderArticleQuantities#created_on
• GroupOrderArticles#updated_on
• GroupOrders#updated_on
• Groups#created_on
• Invoices#paid_on
• Tasks#created_on and Tasks#updated_on
• Users#created_on
• Articles#shared_updated_on

Do we want to rename them all?

[paroga]

i'd go for that. i see no reason for cooking our own soup