Provide more/better configurability for CDN file formats

Problem/Motivation

The module config allows admins to specify a default file format to use for all CDN URLs. This currently presents the following issues:

1. There is no "None" or "N/A" option, so admins are forced to choose a default format unless they want to write code.
2. GIF images are never converted to other formats, by design, but this is not specified in the UI.

Steps to reproduce

See above.

Proposed resolution

Improve admin UI and the logic used when deciding whether/how to apply the default file format setting.

Remaining tasks

Update code and/or UI.

User interface changes

Changes to "CDN" section of admin page.

API changes

Stream wrapper external URL generator will potentially return URLs with different file extensions than before.

Data model changes

None.