Give priority to ATOM to expose WebSub.

Hello,
The ATOM feed contains the hub declaration for WebSub / PubSubHubbub, but the RSS / RDF / JSON versions do not.
Currently, the ATOM version is declared last.
RSS/ATOM clients typically pick whichever version comes first, and will thus not see the WebSub feature.
I therefore suggest putting the ATOM version first, as it is more feature-rich than the other versions.

Clients not compatible with ATOM would not pick it anyway due to the different type attribute.

See same PR for Mastodon: https://github.com/tootsuite/mastodon/pull/9302
and WebSub tester https://websub.rocks/publisher
Status Job ID Name Coverage
  Build
canceled #331
build

canceled #328
build

 
  Test
canceled #330
allowed to fail
code_quality

canceled #329
test