Change priority for S3 Uploads loader

[rmccue]

Currently, S3's init loads on plugins_loaded at priority 10. This means uploads aren't correctly set up prior to this, i.e. if alphabetically-earlier plugins run callbacks on plugins_loaded.

We should run this at a much earlier priority, such as 0, since all it does is add further hooks.

Acceptance criteria:

• Calling wp_upload_dir() from a plugin on the plugins_loaded (default priority) hook should return the S3 URL

[rmccue]

Just ran into this on a second project, in this case caused by AMP running at priority -9223372036854775808. 🙄

[rmccue]

Filed the latter at ampproject/amp-wp#6499