Add base capture animation

[leonhard-s]

Upon ownership change, bases should instantly switch to white background before fading to their new ownership colour.

Very much just polish, but should be simple enough via a custom CSS animation.

[leonhard-s]

CSS animations do not work for this application as we must re-append SVG children on hover to fix the outline draw order, which causes the animations to fire more than once. Workarounds include doubling up the SVG layer to prevent this, or using a JS-controlled animation combined with transitions, neither of which is particularly elegant.