Prompts me to update to a version not available with Flatpak

I have the following version of this Flatpak installed:

```
wjt@camille:~$ flatpak info org.learningequality.Kolibri

Kolibri - Offline education technology platform

ID: org.learningequality.Kolibri
Ref: app/org.learningequality.Kolibri/x86_64/stable
Arch: x86_64
Branch: stable
Version: 1.0
License: MIT
Origin: flathub
Collection: org.flathub.Stable
Installation: system
Installed: 359,0 MB
Runtime: org.gnome.Platform/x86_64/3.36
Sdk: org.gnome.Sdk/x86_64/3.36

Commit: 15c8974f1a788940a296d761afbb43d164f3260e1613b59075eba6efa52997be
Parent: f4604ad83d34eff6bd8de93514eb7d9ebae8331ec621e8a1f3a7dd9a758e538b
Subject: Update kolibri-installer-gnome (21b1a3ef)
Date: 2020-05-30 05:03:55 +0000
```

(Note Version: 1.0 which I think applies to the kolibri-installer-gnome version number – the version of Kolibri itself is Kolibri 0.13.3a0.dev0+git.62.g5d05c61b.)

Whenever I start the app and log in (grr), I get the following prompt:


As a user of the Flatpak, I can't take action based on this notification. At a minimum, the Flatpak package should disable Kolibri's update check.

Ideally, kolibri-installer-gnome would detect that it is within a Flatpak and use the [Flatpak UpdateMonitor API](https://flatpak.github.io/xdg-desktop-portal/portal-docs.html#gdbus-org.freedesktop.portal.Flatpak.UpdateMonitor) to detect when an update is available and prompt the user to apply the update and restart the app.

Start by tracing the update-check path in kolibri-installer-gnome and reviewing how the Flatpak package launches the app. The minimum done condition is that Flatpak users no longer receive an unusable Kolibri update prompt; the issue also describes Flatpak UpdateMonitor integration as the ideal outcome.

Written by the indexing model from the issue text.