Make rss/atom identifier globally unique (go-gitea#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=&go-gitea#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=&go-gitea#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=&go-gitea#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=&go-gitea#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 go-gitea#21542

Signed-off-by: Yarden Shoham <hrsi88@gmail.com>
Co-authored-by: Lauris BH <lauris@nix.lv>

routers/web/feed/convert.go

@@ -241,7 +241,7 @@ func feedActionsToFeedItems(ctx *context.Context, actions activities_model.Actio
 				Name:  act.ActUser.DisplayName(),
 				Email: act.ActUser.GetEmail(),
 			},
-			Id:      strconv.FormatInt(act.ID, 10),
+			Id:      fmt.Sprintf("%v: %v", strconv.FormatInt(act.ID, 10), link.Href),
 			Created: act.CreatedUnix.AsTime(),
 			Content: content,
 		})