Guild banners, and notifications that actually arrive

Branding improvements for guilds, and fixes to notifications under the hood.

🏰 Guild banners

The banner you upload for your guild now appears on the Guilds screen. Guild pages were also sometimes showing incorrect game counts, now fixed.

🔔 Notifications

Cancelling a game now correctly notifies the people who signed up for it, as do updates to the session-scheduling poll tool.

🎲 At the table

Measurement templates on the virtual tabletop now snap to grid intersections rather than tile centres. Fixed some bugs with maps not always loading correctly.

🐛 Other Fixes & Improvements

• Marking a player absent for a date keeps that absence on that date if the schedule moves.
• The "Add Discord bot" buttons correctly point at the bot.
• Recording and transcription reliability work: recordings that stalled now recover on their own, and some improvements to transcription stability.
• Card disputes are recorded properly in our logs as well as Stripe's.