Issues: support setting issue template field values with query

In Blender this is used to report a bug from inside the application, with
some fields automatically filled in.

routers/web/repo/issue.go

@@ -783,6 +783,14 @@ func setTemplateIfExists(ctx *context.Context, ctxDataKey string, possibleFiles
 		ctx.Data[ctxDataKey] = template.Content
 
 		if template.Type() == api.IssueTemplateTypeYaml {
+			// Replace field default values by values from query
+			for _, field := range template.Fields {
+				fieldValue := ctx.FormString("field:" + field.ID)
+				if fieldValue != "" {
+					field.Attributes["value"] = fieldValue
+				}
+			}
+
 			ctx.Data["Fields"] = template.Fields
 			ctx.Data["TemplateFile"] = template.FileName
 		}