Bug 854315 - Display a banner to warn that validation of signatures is not supported in PDF files r=bdahl DONTBUILD

Thanks to PR: mozilla/pdf.js#13214 signatures in PDF will be displayed but they won't be validated.
So display a banner to warn user about that.

Differential Revision: https://phabricator.services.mozilla.com/D111540

Author: calixteman

toolkit/components/pdfjs/content/PdfStreamConverter.jsm

@@ -583,6 +583,8 @@ class ChromeActions {
     var message;
     if (featureId === "forms") {
       message = getLocalizedString(strings, "unsupported_feature_forms");
+    } else if (featureId === "signatures") {
+      message = getLocalizedString(strings, "unsupported_feature_signatures");
     } else {
       message = getLocalizedString(strings, "unsupported_feature");
     }

toolkit/components/telemetry/Histograms.json

@@ -10114,7 +10114,7 @@
     "releaseChannelCollection": "opt-out",
     "bug_numbers": [1630844, 1666979, 1689264],
     "kind": "categorical",
-    "labels": ["unknown", "forms", "javaScript", "smask", "shadingPattern", "font", "errorTilingPattern", "errorExtGState", "errorXObject", "errorFontLoadType3", "errorFontState", "errorFontMissing", "errorFontTranslate", "errorColorSpace", "errorOperatorList", "errorFontToUnicode", "errorFontLoadNative", "errorFontGetPath"],
+    "labels": ["unknown", "forms", "javaScript", "signatures", "smask", "shadingPattern", "font", "errorTilingPattern", "errorExtGState", "errorXObject", "errorFontLoadType3", "errorFontState", "errorFontMissing", "errorFontTranslate", "errorColorSpace", "errorOperatorList", "errorFontToUnicode", "errorFontLoadNative", "errorFontGetPath"],
     "description": "The reason the fallback bar was shown."
   },
   "PDF_VIEWER_FALLBACK_ERROR": {