Disabled Sharp's internal cache to improve memory usage

refs https://sharp.pixelplumbing.com/api-utility#cache

- Sharp has a 50MB cache by default, used within libvips, to increase
  the performance of transforming images
- this isn't relevant to us because we should never be optimizing the
  same image as we check if the optimized image already exists
- I presume there is also some extra overhead of using the cache because
  the memory doesn't seem to grow by 50MB
- the memory usage comparison in Ghost is pretty drastic - uploading 10
  images in serial w/ jemalloc:
  - with cache (default) = peak of 480MB, settles down to 330MB
  - disabling cache = peak of 270MB, settles down to 161MB
- this commit disables the cache
- also adds stubbing for the function in tests

ghost/image-transform/lib/transform.js

@@ -55,6 +55,10 @@ const unsafeResizeFromPath = (options = {}) => {
  */
 const unsafeResizeFromBuffer = (originalBuffer, {width, height} = {}) => {
     const sharp = require('sharp');
+
+    // Disable the internal libvips cache - https://sharp.pixelplumbing.com/api-utility#cache
+    sharp.cache(false);
+
     return sharp(originalBuffer)
         .resize(width, height, {
             // CASE: dont make the image bigger than it was

ghost/image-transform/test/transform.test.js

@@ -66,6 +66,8 @@ describe('Transform', function () {
                 return sharpInstance;
             });
 
+            sharp.cache = sinon.stub().returns({});
+
             testUtils.modules.mockNonExistentModule('sharp', sharp);
         });