Update to old beta channel saying app has moved

[gnprice]

Now that the app has launched, we're publishing future releases to the main Zulip app's listings in the Play Store and App Store, and no longer to the listings (in the Play Store and TestFlight) that were for the Flutter app as a beta.

People who were using the Flutter beta will therefore want to install the main Zulip app and switch to that, in order to keep getting new features and bugfixes. For those who don't see the announcement through other channels, it would be good to post it in the app itself.

Implementation

I think the shape this would take would be a small branch atop the last pre-launch beta version, v0.0.33, adding a dialog box using code similar to that in #1590.

Spec

Two questions we'll need to pin down before shipping this:

• What should the text of the dialog box be?
• When should the dialog appear? Options include just once; or on every startup of the app; or some more-complicated description in between.

[chrisbobbe]

• When should the dialog appear? Options include just once; or on every startup of the app; or some more-complicated description in between.

I vote "on every startup of the app". It's easiest to implement, and I think the goal is to be assertive but stop short of a rug-pull where we just disable all the app's features.

[alya]

Yep, that plan sounds good to me.