Allow adding entire log files using drag and drop

[KurtThiemann]

This change allows users to drag and drop an entire log file onto the page without having to open it and manually copy its contents. A button to select a local file using a traditional file select dialog is also added, since using drag and drop does is not possible on most mobile devices. GZIP compressed log files (.gz) are unpacked automatically using the DecompressionStream API (if available) or fflate.

(not sure about the look for the button yet)