Mobile welcome screen

[wbazant]

Having a landing page could make the web app feel more like an app, and give us a place to make the right first impression.

The welcome screen should be accessible for the / path when there no credentials in localStorage: if someone can be signed in, redirect them to /map as now.

Proposed contents, from bottom to top:

• Logo
• Tiny bit of text about who we are
• Select language
• Sign in | Sign up
• "Continue without account"
• Tiny bit of text about the site being free to use, and you can donate

The donation text can eventually become a text about buying the app to support us.

[wbazant]

Also: explain that the old site is still available under old.fallingfruit.org, in a concise sentence that will go away at some point?

If we find it good to have a welcome screen, it should probably also appear on desktop.