Improve image and pdf contents rendering

The Software Heritage browse web application currently supports the rendering of images (in the README panel inside a directory view or directly in a content view) but also PDF files (only in a content view). Nevertheless, when those images or pdfs have a large size, they are not adjusted to fit the main content panel of the web application (see for instance the rendering of this README, this image or this pdf).

In order to offer a better rendering of those contents, their size should be adjusted when it overflows the main content panel.

For the image adjustements, the CSS object-fit property should be used. For the pdf size adjustements, this should be feasible using the pdf.js API.

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Migrated from T1900 (view on Phabricator)