Add a splash screen to Tails persistence assistant

Originally created by @tchou on #10115 (Redmine)

During test session we felt that the Persistence Assistant was taking some time to start. Not a big deal, but a few second. Maybe we could have a splash screen while the software is lunching ?

Related issues

Related to #6876 (closed)
Related to #6877 (closed)
Related to #15827 (closed)

Edited May 15, 2020 by tchou

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