[regression] Restore the view mode of flag links

Problem/Motivation

Regression since 5.1.0 (5.0.x is not affected): flag links lose the view mode that #3049155: Disclose flagging view mode in flag theming passes to them, for the flag URLs and the view mode theme suggestions. Two changes cause it:

• #3151909: Flag does not render in node Preview rerolled a patch older than #3049155: Disclose flagging view mode in flag theming and accidentally dropped the view mode from the lazy builder in hook_entity_view(), so links with "Display link as field" always use "default".
• #3555233: Use AJAX submit when confirmation form is modal re-renders the link after a "Confirm Form" is submitted in a dialog, but hardcodes the "full" view mode, although the confirm form URL carries the original one.

Steps to reproduce

1. Create a node flag with the "Confirm Form" link type, form behavior "Modal dialog" and "Display link as field".
2. Promote a node and visit /node: the flag link URL contains view_mode=default, not teaser.
3. Click the link and confirm: the re-rendered link contains view_mode=full.

Proposed resolution

• Pass the view mode to the lazy builder again, and to the link of new entities (preview).
• In the confirm forms' AJAX submit handler, read the view mode from the request query, falling back to "full".

Sites that added a flag--default.html.twig template since 5.1.0 to target links displayed as a field must rename it to the actual view mode.

Remaining tasks

Review.

User interface changes

None.

API changes

None.

Data model changes

None.