Update build information

Add the command necessary to add ppa repository in Ubuntu

Refs #3757
Merge pull request #3762

93c3e21 Re-enable UPnP by default in gitian builds (Wladimir J. van der Laan)
Merge pull request #3758

f8a7b50 Update build information (Drak)
qt: Modernize 'confirmed' terminology in shown tx status

These days we regard transactions with one confirmation to be
'Confirmed'.
Waiting for 6 confirmations is a recommendation but should not
keep the transaction shown as unconfirmed.

Misc code sanity:

- Merge maturity/status enums, they had become completely disjunct
- 'confirmed' flag is now called 'countsForBalance' for clarity