Clarify the operation of online and offline installers, and how users
can manage the licenses during installation.

Update the license file-related information in 'Building from Sources'
docs.

Update the Qt Account portal address.

Change-Id: I0437aad9000f15f2c3ad4fb9c0e9bd7096619b3c
Reviewed-by: Sami Makkonen <sami.makkonen@qt.io>
Reviewed-by: Tuukka Turunen <tuukka.turunen@theqtcompany.com>
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>