WebSub: ATOM before RSS (#9302)

Hello,
The ATOM feed contains the hub declaration for WebSub, but the RSS
version does not.
RSS/ATOM readers will 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 its RSS counterpart is.

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

A more complicated alternative would be to declare the WebSub feature in
the RSS version as well, using something like the following code, and
ensuring that clients subscribed to the RSS version would receive PuSH
updates just like those subscribed to the ATOM version.

````xml
<rss version="2.0" xmlns:webfeeds="http://webfeeds.org/rss/1.0"
xmlns:atom="http://www.w3.org/2005/Atom">
<channel>
<atom:link rel="self" type="application/rss+xml"
href="https://diaspodon.fr/users/test.rss"/>
<atom:link rel="hub" href="https://diaspodon.fr/api/push"/>
</channel>
</rss>
```

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