fix(seed): Add tests, solution, and image for Project Euler problem 15 (#16626)

Added more tests, a solution, and an image to make the description more
clear. The image is from Project Euler but is rehosted on imgur.

Amend: Edited the seed so that the challenge function in the seed is called only once.

BREAKING CHANGE: None

seed/challenges/08-coding-interview-questions-and-take-home-assignments/project-euler-problems.json

@@ -693,21 +693,24 @@
       "type": "bonfire",
       "title": "Problem 15: Lattice paths",
       "tests": [
-        "assert.strictEqual(euler15(), 137846528820, 'message: <code>euler15()</code> should return 137846528820.');"
+        "assert.strictEqual(latticePaths(4), 70, 'message: <code>latticePaths(4)</code> should return 70.');",
+        "assert.strictEqual(latticePaths(9), 48620, 'message: <code>latticePaths(9)</code> should return 48620.');",
+        "assert.strictEqual(latticePaths(20), 137846528820, 'message: <code>latticePaths(20)</code> should return 137846528820.');"
       ],
-      "solutions": [],
+      "solutions": ["function latticePaths(gridSize) {\n  let paths = 1;\n\n  for (let i = 0; i < gridSize; i++) {\n    paths *= (2 * gridSize) - i;\n    paths /= i + 1;\n  }\n  return paths;\n}"],
       "translations": {},
       "challengeSeed": [
-        "function euler15() {",
+        "function latticePaths(gridSize) {",
         "  // Good luck!",
         "  return true;",
         "}",
         "",
-        "euler15();"
+        "latticePaths(4);"
       ],
       "description": [
         "Starting in the top left corner of a 2×2 grid, and only being able to move to the right and down, there are exactly 6 routes to the bottom right corner.",
         "",
+        "<img class=\"img-responsive center-block\" alt=\"a diagram of 6 2 by 2 grids showing all the routes to the bottom right corner\" src=\"https://i.imgur.com/1Atixoj.gif\">",
         "",
         "How many such routes are there through a 20×20 grid?"
       ]