Point at the Coffey-Labs organisation

The repositories moved off LINUXexpert-org. The old URLs redirect, so
nothing was broken, but a redirect is not a correct address to publish.

The SOURCE_URL defaults matter most: the AGPL asks whoever runs a
modified version to offer that version's source, and the sign-in page and
About screen show this link. It is in four places that have to agree --
the compose file, .env.example, the server default and the web fallback.

The rest is documentation and issue links.

web/src/lib/source.ts

@@ -5,4 +5,4 @@
  * source. The server says where its own lives, via SOURCE_URL; this is only the
  * fallback for when it has not been asked yet, or has nothing to say.
  */
-export const DEFAULT_SOURCE_URL = "https://github.com/LINUXexpert-org/ihasmail";
+export const DEFAULT_SOURCE_URL = "https://github.com/Coffey-Labs/ihasmail";