Escape Project Metadata in the Project Page and Restrict Accessory Opening

The Project page rendered the model and profile name, author, description
and accessory file names from the 3MF as live HTML. Names, authors and file
names are now set as text, and the file list is built from DOM nodes with
bound click handlers instead of concatenated markup. Descriptions can
legitimately carry rich-text HTML, so they are rebuilt from an inert
DOMParser document, keeping only plain formatting tags, http(s) links and
http(s) images, with every other attribute dropped.

Opening an accessory from the page now only launches regular files that
lie inside the project's extracted auxiliary directory. The containment
check is a new libslic3r helper, is_absolute_path_within_root, built on
is_path_within_root so symlinks leading out of the root are rejected too.

tests/libslic3r/test_utils.cpp

@@ -322,3 +322,48 @@ TEST_CASE("is_symlink_target_within_root rejects a target that passes through a
     CHECK(is_symlink_target_within_root("link", "in/lib.so", root));
 }
 #endif
+
+TEST_CASE("is_absolute_path_within_root accepts only entries inside the root", "[utils]") {
+    namespace fs = boost::filesystem;
+    ScopedTemporaryDir outer;
+    const fs::path root = outer.path() / "Auxiliaries";
+    fs::create_directories(root / "Others");
+    const fs::path inside = root / "Others" / "note.txt";
+    const fs::path outside = outer.path() / "secret.txt";
+    std::ofstream(inside.string()) << "inside";
+    std::ofstream(outside.string()) << "outside";
+
+    SECTION("a file inside the root") {
+        REQUIRE(is_absolute_path_within_root(inside, root));
+    }
+    SECTION("a path inside the root whose file does not exist yet") {
+        REQUIRE(is_absolute_path_within_root(root / "Others" / "missing.txt", root));
+    }
+    SECTION("the root itself") {
+        REQUIRE_FALSE(is_absolute_path_within_root(root, root));
+    }
+    SECTION("a parent-directory escape spelled under the root") {
+        REQUIRE_FALSE(is_absolute_path_within_root(root / "Others" / ".." / ".." / "secret.txt", root));
+    }
+    SECTION("an absolute path elsewhere") {
+        REQUIRE_FALSE(is_absolute_path_within_root(outside, root));
+    }
+    SECTION("a sibling directory sharing the root's name as a prefix") {
+        const fs::path sibling = outer.path() / "Auxiliaries2" / "note.txt";
+        REQUIRE_FALSE(is_absolute_path_within_root(sibling, root));
+    }
+    SECTION("a relative path") {
+        REQUIRE_FALSE(is_absolute_path_within_root(fs::path("Others") / "note.txt", root));
+    }
+    SECTION("an empty path") {
+        REQUIRE_FALSE(is_absolute_path_within_root(fs::path(), root));
+    }
+#ifndef _WIN32
+    // Creating symlinks on Windows needs elevated rights or developer mode.
+    SECTION("a symlink inside the root that points outside") {
+        const fs::path link = root / "Others" / "link.txt";
+        fs::create_symlink(outside, link);
+        REQUIRE_FALSE(is_absolute_path_within_root(link, root));
+    }
+#endif
+}