Make evidence captions configurable (required vs. optional)

[KevinRBriggs]

Is your feature request related to a problem? Please describe.
When uploading evidence, the caption field is always required. In some workflows — particularly when embedding screenshots that are self-explanatory within the finding's narrative — a caption adds no value but is still mandatory. When a caption is left blank (e.g., by directly editing the database or a future change), the DOCX report writer still renders a dangling "Figure N –" line with no caption text after it, which looks like a formatting error in the delivered report.

Describe the solution you'd like
A "Require Evidence Captions" toggle to the Admin Panel under Commandcenter > Global Report Configuration > Captions, defaulting to enabled so existing behavior is preserved.

When an admin disables it:

• The EvidenceForm treats the caption field as optional, with updated placeholder text and help text indicating it can be left blank.
• The DOCX report writer suppresses the figure caption paragraph entirely for evidence that has no caption, so no "Figure N –" artifact appears in the generated report.

This keeps the feature opt-in and gives teams the flexibility to decide whether captions are part of their reporting workflow.

Describe alternatives you've considered

• Simply making captions always optional (no toggle). This would change existing behavior for all installations and could surprise teams that rely on captions being enforced. A configurable approach avoids that.
• A per-evidence "show caption" checkbox. This adds UI complexity to every evidence upload for a setting most teams would want consistent across all evidence. A single global toggle is simpler.

Additional context
I have a working implementation and would be happy to submit a PR. The change touches the ReportConfiguration model (new BooleanField + migration), the admin fieldset, EvidenceForm, and the DOCX report writer's figure caption rendering. Tests are included covering both configuration states.