Show a welcome dialog on upgrading from legacy app

[gnprice]

When this app launches for the first time in an install that previously held the legacy app (following the launch, cf #715), we should show a brief dialog box to acknowledge the obvious that the app will look quite different, and to offer more information.

Here's text for the dialog box (written by @alya):

Welcome to the new Zulip app!

You’ll find a familiar experience in a faster, sleeker package. Check out the announcement blog post!

[Let’s go]

The link will be to a URL under https://blog.zulip.com/… which we've picked in advance, and which will redirect to the blog post once it goes up. (The blog post won't yet be up when we're preparing the release build, so its actual URL won't yet be final, hence the indirection.)

A key step in this issue is to determine whether the app was upgraded from the legacy app. We'll have that information as a side effect of #1070, so I expect to implement this issue next after that one.

[chrisbobbe]

#1285 blocks doing links in prose, if it's translated. (In fact I'm not sure offhand how to make a link in un-translated text, except in the context of Zulip message content, and we don't have #488 yet.)

We can give the dialog an extra action button though. (For the implementation, maybe showSuggestedActionDialog does what we'd want?)

[gnprice]

Hmm true.

Given that, I think the most expedient thing is if the link is a whole paragraph of its own. Then it can be its own Text widget, wrapped in a gesture detector with an onPressed that opens the URL. (And a separate string for translation.)

To fit that constraint, how about:

You’ll find a familiar experience in a faster, sleeker package.

Check out the announcement blog post!