Add DeepL bulk action to the page tree view

[charludo]

Motivation

We currently lack a way of bulk-translating pages into a supported language, even though the same feature exists for POIs and events. While a more in-depth translation manager is still being debated, we want to at least be able to offer a bulk action for this that is consistent with the one for POIs/events.

Proposed Solution

• enable the bulk action in the page_tree template (see existing implementations for other content types)
• add a confirmation/summary popup containing all relevant information (pages which will be translated, pages which cannot be translated due to poor hix value (only if source language is German!), remaining budget and how much of it will be used by the bulk action), as well as the option to change source and target languages (see mockup by @JoeyStk below)
• if the HIX value calculation is disabled for a page (hix_ignored=true), the page should not be translated via DeepL (see Expert mode: Ability to mark/umark "Ignore Textverständlichkeitsindex" on a content page #2075)
• if the budget allows for the bulk action, enable the confirmation button

Alternatives

• automatically un-select all low-HIX pages when this bulk action is selected

Additional Context

Blocked by:

• Store the HIX value of pages in the database #2101

Design Requirements

(update, not in mockup: move budget section up above the untranslatable pages)