Use better device name

[aaronraimist]

Currently when you login to a new device it is just called Mobile.

Here is the API to get the name of the device that the user sets: https://developer.apple.com/documentation/uikit/uidevice/1620015-name

[aaronraimist]

I guess maybe Riot iOS use to do this: #910

Not sure what the best compromise is but I prefer the old behavior

[aaronraimist]

Riot's Mac App puts the name of your computer as part of the device name

[manuroe]

@nadonomy, could we add this to UX requirements for the new registration flows, please?

What we want to fix here is how to define the public name of the user's device displayed in the e2e dialogs.
This name is currently automatically computed by the app.

For privacy reason, we removed in #910 the old behavior that reused the name the user gives to their device but that does not please to everyone.

We could have a screen in the e2e setup of the registration flow that asks the user what public name they want to use for the device.