Multiple thumbnail support

[iambibhas]

It should be possible to generate thumbnails in multiple format. Right now the thumbnail extension is taken from a hardcoded value in ALLOWED_MIMETYPES dict. It needs to be made dynamic. And extension needs to be saved in db along with the file name. This also relates to #69.

[jace]

#70 "GIF to video" describes a related issue.

[jace]

Here's a new one: SVG previews of bitmap images. In this case because clarity is lost, this image is meant to be used as a loading placeholder instead of a representative thumbnail.

We need a mechanism by which Imgee can provide SVG thumbnails that can be loaded inline into a client app's page, rather than in additional requests (which can just get the final version to be displayed).