Check for new releases in Slack release-notes page

[epontan]

It seems that the release-notes page1 is more up-to-date than the download page2 for the Linux client. Use that instead as source for checking if new releases are available.

Fixes #218

[flathubbot]

Started test build 65615

[flathubbot]

Build 65615 successful To test this build, install it from the testing repository:

flatpak install --user https://dl.flathub.org/build-repo/48256/com.slack.Slack.flatpakref

[flathubbot]

Started test build 69678

[flathubbot]

Build 69678 successful To test this build, install it from the testing repository:

flatpak install --user https://dl.flathub.org/build-repo/52329/com.slack.Slack.flatpakref

[travier]

CC @hfiguiere