Documentation: Update $wgPasswordDefault and BcryptPassword

Slight syntax code change for $wgPasswordDefault in DefaultSettings.php
and fixed reference to global in BcryptPassword.php.

Change-Id: I8d1d12c09ecd2f422f21a586e948f314e29fa605
test: coverage for wfWikiID()

Basic coverage test, note that $wgDBprefix is used as a suffix!

Bug: 68231
Change-Id: Ic454ce38672b1f18db2a7f2f46c50bcfc5ee83d0
Merge "Documentation: Update $wgPasswordDefault and BcryptPassword"
Merge "test: coverage for wfWikiID()"