Describe the bug
Some sites may have blavatar URLs returned by the xpost endpoint that are malformed, e.g. are wrapped in an <img> tag.

To Reproduce

1. Find a site that can Xpost to another site that has a blavatar URL that contains an <img> tag
2. Notice that Xposts fail to load on those sites

Expected behavior
We should handle these situations gracefully and display a placeholder image for those sites.

Smartphone (please complete the following information):

• Device: iPhone
• OS: iOS 11
• Version develop branch