[gui] intro: enable pruning by default unless disk is big

Summary:
Color the text "GB needed for full chain" orange for nodes that barely have enough space.

This concludes backport of Core PR16714 - part 5 of 5
Commit 9924bce317b96ab0c57efb99330abd11b6f16b9a
Depends on D7955

Test Plan:
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