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 default name no longer than 64 bytes.

Test Plan:

• Used the {image ...} example from the Discourse report locally; got an image with a truncated name.
• Used a normal {image ...}, got an image file with a normal name.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D19353