Rename Tails Greeter to be more plain

Originally created by @sajolida on #15122 (Redmine)

During the expert review, “Tails Greeter” wasn’t an intuitive name. She would have preferred “Tails Welcome Screen” or something similar.

I understand that “Greeter” comes from the backend technology from GNOME.

We could check how other projects are calling such screens.

Edited May 21, 2020 by sajolida

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