Make rss/atom identifier globally unique (#21550)
Make rss/atom identifier globally unique (#21550)

This field should be globally unique.

[RSS
reference](https://www.rssboard.org/rss-specification#ltguidgtSubelementOfLtitemgt).

### Before
```xml
    <item>
      <title>abc opened issue &lt;a href=&#34;https://3000-yardenshoham-gitea-3pzuhkduf6t.ws-eu72.gitpod.io/abc/defg/issues/1&#34;&gt;abc/defg#1&lt;/a&gt;</title>
      <link>https://3000-yardenshoham-gitea-3pzuhkduf6t.ws-eu72.gitpod.io/abc/defg/issues/1</link>
      <description>1#Colors</description>
      <content:encoded><![CDATA[<p><code>#FF0000<span class="color-preview" style="background-color: #FF0000"></span></code></p>
]]></content:encoded>
      <author>abc</author>
      <guid>2</guid>
      <pubDate>Mon, 17 Oct 2022 16:06:08 +0000</pubDate>
    </item>
    <item>
      <title>abc created repository &lt;a href=&#34;https://3000-yardenshoham-gitea-3pzuhkduf6t.ws-eu72.gitpod.io/abc/defg&#34;&gt;abc/defg&lt;/a&gt;</title>
      <link>https://3000-yardenshoham-gitea-3pzuhkduf6t.ws-eu72.gitpod.io/abc/defg</link>
      <description></description>
      <author>abc</author>
      <guid>1</guid>
      <pubDate>Mon, 17 Oct 2022 16:05:43 +0000</pubDate>
    </item>
```
### After
```xml
    <item>
      <title>abc opened issue &lt;a href=&#34;https://3000-yardenshoham-gitea-3pzuhkduf6t.ws-eu72.gitpod.io/abc/defg/issues/1&#34;&gt;abc/defg#1&lt;/a&gt;</title>
      <link>https://3000-yardenshoham-gitea-3pzuhkduf6t.ws-eu72.gitpod.io/abc/defg/issues/1</link>
      <description>1#Colors</description>
      <content:encoded><![CDATA[<p><code>#FF0000<span class="color-preview" style="background-color: #FF0000"></span></code></p>
]]></content:encoded>
      <author>abc</author>
      <guid>2: https://3000-yardenshoham-gitea-3pzuhkduf6t.ws-eu72.gitpod.io/abc/defg/issues/1</guid>
      <pubDate>Mon, 17 Oct 2022 16:06:08 +0000</pubDate>
    </item>
    <item>
      <title>abc created repository &lt;a href=&#34;https://3000-yardenshoham-gitea-3pzuhkduf6t.ws-eu72.gitpod.io/abc/defg&#34;&gt;abc/defg&lt;/a&gt;</title>
      <link>https://3000-yardenshoham-gitea-3pzuhkduf6t.ws-eu72.gitpod.io/abc/defg</link>
      <description></description>
      <author>abc</author>
      <guid>1: https://3000-yardenshoham-gitea-3pzuhkduf6t.ws-eu72.gitpod.io/abc/defg</guid>
      <pubDate>Mon, 17 Oct 2022 16:05:43 +0000</pubDate>
    </item>
```

* Fixes #21542

Signed-off-by: Yarden Shoham <0b3acf1045a60ccc22a11aaedae8f612108f28a6@gmail.com>
Co-authored-by: Lauris BH <f3041c4c693c07149804ea655e75ee8bb856bd26@nix.lv>
go-gitea/gitea,go-gitea/gitea,go-gitea/gitea