More security/robustness updates, plus a quality-of-life improvement for avatar uploads.

We now impose strict Content-Security-Policy headers site-wide, which hopefully will go unnoticed, but will reduce attack surface and mitigate some low-effort exploits. Suspicious activity on the registration page will impose an automatic IP ban, and CAPTCHA now required, since it's a nice modern accessible one now.

A regression on the Credit Multiplier page has been fixed, so credit submissions should work again. Additionally, player and guild avatar images now accept up to 2048x2048 images, which we will downscale to 1024x1024 for the master image, from which we will create various thumbnail sizes for use throughout the game. Slightly tidier setup than before.

Any problems at all, please post in the forum and we'll be sure to address them.