Include all bridges in tarballs

Currently, two "demo" and "example" bridges are excluded from GitHub's
autogenerated tarballs. As I argued, those files can still be helpful
for integration tests, as they are run in NixOS and don't need internet
access or depend on the availability of external services [1].

Additionally, the official docker image builds from the checkout so it
includes those bridges when users use containers or a git checkout
compared to tarballs. This commit therefore unifies the list of
available bridges between deployment methods.

[1] https://github.com/NixOS/nixpkgs/blob/master/nixos/tests/web-apps/rss-bridge.nix#L20

.gitattributes

@@ -47,8 +47,6 @@ phpcs.xml                               export-ignore
 phpcompatibility.xml                    export-ignore
 tests/                                  export-ignore
 cache/.gitkeep                          export-ignore
-bridges/DemoBridge.php                  export-ignore
-bridges/FeedExpanderExampleBridge.php   export-ignore
 
 ## Composer
 #