nixos/nextcloud: update nginx config

Please note that I didn't use the current nginx config from the
administration manual as this would've broken ACME challenges[1].

Also added a fix for Microsoft clients.

[1] https://github.com/nextcloud/documentation/pull/5825#issuecomment-783977761

(cherry picked from commit 797721423c)

nixos/modules/services/web-apps/nextcloud.nix

@@ -605,6 +605,14 @@ in {
               access_log off;
             '';
           };
+          "= /" = {
+            priority = 100;
+            extraConfig = ''
+              if ( $http_user_agent ~ ^DavClnt ) {
+                return 302 /remote.php/webdav/$is_args$args;
+              }
+            '';
+          };
           "/" = {
             priority = 900;
             extraConfig = "rewrite ^ /index.php;";
@@ -623,6 +631,9 @@ in {
               location = /.well-known/caldav {
                 return 301 /remote.php/dav;
               }
+              location ~ ^/\.well-known/(?!acme-challenge|pki-validation) {
+                return 301 /index.php$request_uri;
+              }
               try_files $uri $uri/ =404;
             '';
           };