Add spells tracker (current / next achievement)

[VeteraNovis]

Modifiy the spellsSection of the statistics page to include a line that
tracks the total spells cast, and the number till the next achievement.

[DanielNoord]

Sorry for taking so long to review this.

Just one comment: how does this work when the Grimoire isn't loaded yet? For example on new saves the Grimoire isn't loaded and the div therefore probably won't be found?

[VeteraNovis]

@DanielNoord Ah, yep that's definitely something I overlooked. I would have to mark it as 0 until the grimoire is unlocked, as it makes sense that no spells could be cast until that point.

[DanielNoord]

@VeteraNovis Can we make it so that it doesn't display at all when the Grimoire isn't unlocked? We will also need to handle the failure of the div lookup gracefully.

[VeteraNovis]

Sounds like a good idea. I'll put some more time into it and update the PR.