Add Firefox Sync service

This pull request is about adding one of the Mozilla services [https://github.com/mozilla-services] to NixOS.  The intent being to make it easier for Firefox users to host their own data as much as possible. First, because this is possible and suggested [https://docs.services.mozilla.com/howtos/run-sync-1.5.html] and this would be nice for NixOS users as well.  Second, because NixOS provide easy ways for making and sharing small system images.

Currently, this work is neither using Apache nor nginx, but this might be an improvement on top of this one.
This work was done by @ametaireau and my-self.

Firefox sync can be enabled by setting the option 
```
services.firefox.syncserver.enable = true;
```

in your NixOS configuration, and by updating `about:config` settings of Firefox to replicate the following line

```
services.sync.tokenServerURI: http://localhost:5000/token/1.0/sync/1.5
```

as explained in the documentation of the `enable` option.
Never seen something like this. Should it be echo '[syncserver]' > ${syncServerSecretFile} instead?