Add support for multipart/form-data conversions

[MrSaints]

Upload-based conversions can now be carried out by
calling a POST request on /convert using the file
parameter.

Example:

curl -F file=@index.html localhost:8080/convert

The output is a binary stream (much like the GET
endpoint).

This change significantly refactors the way a conversion
target / source is handled. When running a conversion,
we now provide an explicit conversion source. Before, we
relied on an implicit source (that we simply assumed a
Converter will handle).

The conversion source also contains the MIME type of
our conversion target. We can extend this to handle a
range of different conversions in the future.

The old util package, and its methods have been
integrated into source.go. All URLs are checked w/
cookie support.

A new CloudConvert method was also introduced for quick
upload-based conversions. In the future, it might be
worth using the inline download method only, and rely
on our own S3 uploader (that way, we won't be passing
our S3 credentials around).

TODO: refactor middleware, and application-wide config.

[MrSaints]

cc: @arachnys/qa

[cedric-cordenier]

@MrSaints an example of an Ethiopian page which Athena seems to have trouble handling - nothing to do with this PR in particular, but I thought you'd be interested!

http://www.ethiopianreporter.com/content/%E1%89%A0%E1%8A%A8%E1%8D%8D%E1%89%B0%E1%8A%9B-%E1%8B%A8%E1%8D%96%E1%88%88%E1%89%B2%E1%8A%AB-%E1%8A%A0%E1%88%98%E1%88%AB%E1%88%A9-%E1%8A%A0%E1%8A%AB%E1%89%A3%E1%89%A2-%E1%8B%AB%E1%88%88-%E1%89%BD%E1%8C%8D%E1%88%AD-%E1%8A%AB%E1%88%8D%E1%89%B0%E1%8D%88%E1%89%B3-%E1%88%98%E1%88%8D%E1%8A%AB%E1%88%9D-%E1%8A%A0%E1%88%B5%E1%89%B0%E1%8B%B3%E1%8B%B0%E1%88%AD%E1%8A%95-%E1%88%9B%E1%88%B5%E1%8D%88%E1%8A%95-%E1%8D%88%E1%89%B0%E1%8A%93-%E1%8A%A5%E1%8A%95%E1%8B%B0%E1%88%86%E1%8A%90-%E1%89%B0%E1%8C%A0%E1%89%86%E1%88%98

[MrSaints]

@cedric-cordenier That's a good catch. I'll bear that in mind.

[MrSaints]

Merged.

IMPORTANT:

• If you're using / relying on Weaver's Go API, please note that it has been significantly changed in this PR;
• If you're running into issues with the latest Docker version (which includes this PR), you can pin your Docker to use the last stable version before this (arachnysdocker/athenapdf:2.1.3 and arachnysdocker/athenapdf-service:pre-multipart)