Add graphical attachment mappings and document removal

[nonprofittechy]

The /al/editor workflow now supports deleting documents from template setup and editing attachment field mappings graphically.

• Remove a document from one bundle, all downloads, or the interview. Deletion removes its attachment, title, declaration, and bundle references while retaining the uploaded template and questions. Custom code that refers to the deleted document still needs review.
• Inspect PDF/DOCX template fields and edit text or conditional Mako expressions with variable autocomplete. Missing PDF mappings are flagged; DOCX implicit variables and dynamic mappings are explained. Complex mappings remain editable in YAML mode.
• Save question edits on screens containing attachments without dropping the attachment YAML. Handle empty collections, blank fields, and explicit YAML null values when adding mappings.
• Preserve comments and surrounding source during mapping edits, check revisions before saving, and handle commented attachments and similarly named documents during deletion.

Validation: 81 focused tests passed, including source preservation and frontend regression coverage. The full commit hooks also passed: Black, mypy, JavaScript formatting/lint/type checks, and pytest. The YAML interview hook had no applicable changed files.

Fixes #1060.
Fixes #1018.