Add nextcloud

Nextcloud will allow us to easily replace google docs with open source
alternatives such as Collabora Online.

Other options considered:
- Privatebin (not for collaborative document editing, we do have an instance at
  https://paste.clicks.codes/)
- Etherpad (seemed to require a nontrivial amount of setup and an old version of
  nodejs)
- Owncloud (looks good too, but Nextcloud is the offering I'm more familiar with
  as I've used instances of it before (and some friends host it!))
- Standalone Collabora Online server (would need some other WOPI host or to use
  filesystem... which, no)
- Cryptpad (I'm less familiar with it, and we don't seem to need the things
  which are its selling points)

Still TODO (for followup changes):
- Integrate with Keycloak (added in I53e33f8c7fa21220e5407d4cd75a705c8e19f9a3)
  for authentication

Change-Id: I8cf10ee09b068eb7d74cd2d5619b509eb9581f8d
diff --git a/flake.nix b/flake.nix
index 838808d..3022dc2 100644
--- a/flake.nix
+++ b/flake.nix
@@ -66,6 +66,7 @@
               ./modules/loginctl-linger.nix
               ./modules/matrix.nix
               ./modules/mongodb.nix
+              ./modules/nextcloud.nix
               ./modules/node.nix
               ./modules/postgres.nix
               ./modules/privatebin.nix