Add viewbox attribute to SVG icons to make them resizable

[adamkudrna]

Some SVG icons lack the viewbox attribute and thus cannot be resized via CSS.

For example, Litecoin is fine:

While Bitcoin is not:

Proposed solution: ensure that viewbox="0 0 32 32" is present in all SVG icons.

(Previously reported as #63.)

[weewoo64]

+1 I need this in my life.

[ivands]

Resizable svg's for the win. Please add it.

[sindresorhus]

PR welcome :)

[ackushiw]

Pull request submitted #275

[sindresorhus]

If anyone wants to tackle this, see the previous attempt and the feedback on that PR: #275