Set the isPermaLink attribute to false in the guid sub-element (#28860)

Our `guid` is not a valid URL so according to the RSS spec the
`isPermaLink` attribute needs to be set to `false`.

Example:
```diff
<item>
  <title>yardenshoham opened issue &lt;a href=&#34;https://3000-yardenshoham-gitea-jqlxjixsez9.ws-us107.gitpod.io/yardenshoham/test/issues/2&#34;&gt;yardenshoham/test#2&lt;/a&gt;</title>
  <link>https://3000-yardenshoham-gitea-jqlxjixsez9.ws-us107.gitpod.io/yardenshoham/test/issues/2</link>
  <description>2#hey</description>
  <content:encoded><![CDATA[2#hey]]></content:encoded>
  <author>yardenshoham</author>
- <guid>355: https://3000-yardenshoham-gitea-jqlxjixsez9.ws-us107.gitpod.io/yardenshoham/test/issues/2</guid>
+ <guid isPermaLink="false">355: https://3000-yardenshoham-gitea-jqlxjixsez9.ws-us107.gitpod.io/yardenshoham/test/issues/2</guid>
  <pubDate>Tue, 16 Jan 2024 18:54:36 +0000</pubDate>
</item>
```

References:
-
https://www.rssboard.org/rss-specification#ltguidgtSubelementOfLtitemgt
- Fixes https://github.com/go-gitea/gitea/issues/28734
- https://github.com/gorilla/feeds/issues/78
- https://github.com/go-gitea/gitea/pull/21550
- https://github.com/gorilla/feeds/pull/107

Signed-off-by: Yarden Shoham <git@yardenshoham.com>

routers/web/feed/convert.go

@@ -244,6 +244,7 @@ func feedActionsToFeedItems(ctx *context.Context, actions activities_model.Actio
 			Title:       title,
 			Link:        link,
 			Description: desc,
+			IsPermaLink: "false",
 			Author: &feeds.Author{
 				Name:  act.ActUser.DisplayName(),
 				Email: act.ActUser.GetEmail(),