Fix the stupid things.

This includes:
 - NuulsUploader.cpp
   - changing upload delay to a #define
   - moving png conversion code to `boost::optional<QByteArray> convertToPng(QImage image)`
   - in uploadImageToNuuls: moving content type definition to a variable, move things around a bit to eliminate an if
   - in upload: adding a comment about `source->hasUrls()` and `source->urls()`, change `uploadQueue.size()` to `!uploadQueue.empty()`
 - ResizingTextEdit.cpp
   - changing #include order

src/util/NuulsUploader.hpp

@@ -16,7 +16,3 @@ void upload(TypedBytes imageData, ChannelPtr channel,
 void upload(const QMimeData *source, ChannelPtr channel,
             ResizingTextEdit &outputTextEdit);
 }  // namespace chatterino
-
-namespace {
-QString getImageFileFormat(QString path);
-}