Add thumbnailing support to media repository for SVGs (SYN-344)

[matrixbot]

Python Image Library can't read SVG so the current PIL based thumbnailing code can't generate a thumbnail for an SVG.

For now we might just want to return the original image since it is already scalable.

(Imported from https://matrix.org/jira/browse/SYN-344)

(Reported by @NegativeMjark)

[matrixbot]

Jira watchers: @NegativeMjark @ara4n

[matrixbot]

this is starting to get annoying given our heavy use of SVGs in vector and similar.

It's bad practice to return the raw SVG (but better than nothing) given if I upload a 2MB SVG it'd suck if everyone ended up downloading a 2MB SVG thumbnail...

-- @ara4n

[matrixbot]

Alternatively we could do something like pyrsvg and http://stackoverflow.com/questions/6589358/convert-svg-to-png-in-python

-- @ara4n

[matrixbot]

Workaround using the original image at 2c838f6. Closing this to hand over to that PR

-- @ara4n

[matrixbot]

had to back this out due to bd77216

-- @ara4n