feat(settings): link the Steam account through Steam

A Steam card in the settings links, verifies or unlinks the Steam
account through "Sign in through Steam". After the way back the page
shows the outcome and refreshes the session, so Core gets the new
Steam ID at once. Registration and settings no longer ask for a typed
Steam ID.

README.md

@@ -34,6 +34,9 @@ To Be Played is split into three repositories:
   game still in the backlog, and one to set the status to 100 % when
   every achievement is unlocked.
 - **Wishlist** with store price and discount.
+- **Steam account** linked in the settings with "Sign in through Steam": the user signs in on Steam and Auth gets
+  the proved Steam ID, never a typed-in one. A Steam ID typed in before shows as "not verified" with a button to
+  verify it; unlinking is possible.
 - **Public profile** at `/u/<display name>`, opt-in in the settings, with perfect games and average completion.
 - **Appearance**: accent color, corner style, wide layout, page background and banner from your own game art.
   Saved in Auth, so it follows you to every device.
@@ -68,6 +71,8 @@ flowchart LR
     Core -->|"OAuth token"| Twitch["Twitch"]
     Core --> Speedrun["speedrun.com API"]
     Browser -.->|"game art"| Images["Steam and IGDB image hosts"]
+    Browser -.->|"Sign in through Steam"| SteamLogin["steamcommunity.com (OpenID)"]
+    Auth -->|"check_authentication"| SteamLogin
 ```
 
 - nginx serves the built app. It has no backend of its own: the browser calls Auth and Core directly