Fix overwriting the same asset filename

[cpirich]

• If you use the Manage Assets dialog to upload a file that already exists in your project, we had a couple of problems, which are now fixed:
  1. The assetListStore.add() function was always appending the asset to the assets array, resulting in the old (replaced) asset and the new asset in the asset list at the same time. We now will first attempt to remove any asset with the same filename before adding to the array.
  2. Due to cache lifetime, you would still see the old image thumbnail. Given that we will have a string timestamp value in the assetListStore after a recent upload, we now pipe that through and use it to create a cacheBustSuffix of the form ?t=1524501023442 in the asset thumbnail

[islemaster]

Looks good!