Discourse Signatures

[jamie.wilson]

I’ve merged the above PR from @falco, so the plugin now officially contains:

• Group-based restriction (signatures_allowed_groups) — server-side enforced
• Category-based display (signatures_show_in_categories)
• First-post-only mode (signatures_first_post_only)
• Signature length cap (signatures_max_length) and image height limit (signatures_max_image_height)
• URL validation hardened to HTTP/HTTPS only, max length reduced to 2048

All defaults are backward-compatible.