Add SUMM.AI budget for updating translations

[timobrembeck]

Motivation

We offer municipalities the option to purchase an extra 10.000 words for SUMM.AI update translations

Proposed Solution

• [ ] Create a new setting SUMM_AI_CREDITS_ADDON to define the amount for the SUMM.AI package, similar to DEEPL_CREDITS_ADDON
• [ ] Add new fields summ_ai_addon_booked, summ_ai_renewal_month and summ_ai_budget_used to the region model, similar to deepl_addon_booked, deepl_renewal_month and deepl_budget_used
• [ ] Whenever the content is translated via the new checkbox in the form, increase the used words budget of the region, see:
• [ ] #2110
• [ ] Show a graph of the used budget in the machine translation settings page

The initial translations via the bulk action or the management command should not drain the region's budget.