nixos/*: normalize link format

make (almost) all links appear on only a single line, with no
unnecessary whitespace, using double quotes for attributes. this lets us
automatically convert them to markdown easily.

the few remaining links are extremely long link in a gnome module, we'll
come back to those at a later date.

nixos/modules/services/development/zammad.nix

@@ -139,7 +139,7 @@ in
           '';
           description = ''
             The <filename>database.yml</filename> configuration file as key value set.
-            See <link xlink:href='TODO' />
+            See <link xlink:href="TODO"/>
             for list of configuration parameters.
           '';
         };