Redraw all structural diagrams

[stulacy]

This is an attempt to redraw the structural diagrams of all species in the MCM.
There are several benefits of doing this:

• Force a uniform aspect ratio, as at the moment larger molecules have larger drawings. This makes it harder to draw consistent layouts and is the main reason each species page doesn't contain a large drawing of its structure
• Use SVG allowing for simpler displaying of drawings at different sizes (i.e. small in the search box but large in each species' page)
• Address any inconsistencies such as the crossed-bond issue mentioned in Redraw structural diagrams #181

This PR contains an initial attempt at using rdkit to achieve this in Python, and contains a comparison of the resultant drawings with those currently used. These drawings need to be assessed for their scientific accuracy as well as any aesthetic changes (reducing the pen thickness for example) before they can be incorporated into the main website.

Addresses #115 and #181