Make uploads content seekable

[Desuuuu]

Uploads currently expose their content as an io.Reader, which makes it hard to read their content multiple times.

The content is either backed by an os.File which is seekable or a bytesReader (in-memory buffer) which can be trivially made seekable.

This PR makes bytesReader implement the io.Seeker interface and changes the type of the field File in Upload to io.ReadSeeker to allow consumers to simply call Seek on the upload's content.

The prevRune field has also been removed from bytesReader since it was unused.

I have:

• Added tests covering the bug / feature (see testing)
• Updated any relevant documentation (see docs)

[coveralls]

Coverage decreased (-0.02%) to 75.085% when pulling 8925842 on Desuuuu:upload-seek into a8f112e on 99designs:master.