Welcome dialog on upgrade from legacy app

[gnprice]

Fixes #1580.

Stacked atop #1588.

I believe this works correctly. Some tests would be nice to write, but are less critical than for #1070 / #1588.

Screenshots

dark	light

[gnprice]

@alya for review on the screenshots. (See thread in #1580 for why the link is slightly different from the original plan.)

[gnprice]

I've pushed a revision that adjusts what URL the link goes to:

  static const String _announcementUrl =
    // TODO(#1580) switch to this URL, which will point to the actual blog post:
    //   'https://blog.zulip.com/flutter-mobile-app-launch';
    // For the moment, that's a 404, so point directly to this pre-announcement:
    'https://chat.zulip.org/#narrow/channel/1-announce/topic/New.20mobile.20app.20launching.20this.20week/near/2194879';

We'll want to change that before the actual launch, once the desired URL works. (Which will be before the actual post is up; it can redirect to that same pre-announcement instead, and change to redirecting to the post once the post is up.)

[gnprice]

CI failure in l10n:

================================================================
Running l10n...
There are uncommitted changes in lib/generated/l10n/zulip_localizations*.dart:

 M lib/generated/l10n/zulip_localizations_de.dart

Aborting, to avoid losing your work.
================================================================

Odd. And that's an uncommitted change, so left around by some previous step.

Seems harmless in itself, so I'll ignore for now given the impending launch. If it recurs it'll be good to debug.

[gnprice]

And updated that URL; the desired URL works now 🙂

[gnprice]

Well, that CI failure repeated the same way after my next few pushes after seeing it. But with the recent push which rebased this and its dependency #1588 past these commits in main:
67af2ed logo: Switch app icons to non-"beta" versions
2ab189d (pr/1577) android, ios: Update app name to "Zulip", from "Zulip beta"
40046c8 (pr/1585) notif: Use a different channel ID from previous values of zulip-mobile
a4a96a1 notif: Use PackageInfo to generate sound resource file URL
e0e51b1 android: Switch app ID to that of the main app
378e187 (pr/1586) ios: Switch app ID to that of the main app
5be0e70 l10n: Add translatable strings from welcome dialog in v30.0.256 release
fec0071 (pr/1591) l10n: Update translations from Weblate.
a6ea882 version: Sync version and changelog from v30.0.256 release

CI now passes 🤷.

Of those, the two commits that touched any files in assets/l10n/ or lib/generated/l10n/ were:
5be0e70 l10n: Add translatable strings from welcome dialog in v30.0.256 release
fec0071 l10n: Update translations from Weblate.

so presumably the reason involved one of those.

[chrisbobbe]

LGTM, thanks, please merge at will! 🚀