[gui] intro: enable pruning by default unless disk is big
Color the text "GB needed for full chain" orange for nodes that barely have enough space.
Unmount the filesystem containing the bitcoin data, to trigger the intro screen to show on startup of bitcoin-qt.
Run src/qt/bitcoin-qt -regtest
In the intro screen, select a data directory on a filesystem with less than 10GB free space.
Verify that the message about the available space is yellow, and that the prune checkbox settings got checked.
Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix
Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix
Differential Revision: https://reviews.bitcoinabc.org/D7956