R1:70056a9072d3

When creating a file by downloading a URI, truncate the length of the default name

Summary:
See <https://discourse.phabricator-community.org/t/embedding-external-images-url-show-error-for-long-urls/1339>.

When we download a file from a URI, we provide a default name based on the URI. However, if the URI is something like `http://example.com/very-very-very-....-long.jpg` with more than 255 characters, we may suggest a name which won't fit into the `name` column of `PhabricatorFile`.

Instead, suggest a…