Merge branch 'master' into adodb-upg-master

Conflicts:
	admin/schema.php
Merge branch 'adodb-upg-master' into 13x
Remove library/adodb
ADOdb library is now a git submodule

A new 'mantis-1.3' branch was created from upstream's master, which is
currently just before release of V5.19, and will be used to track
MantisBT-specific changes for 1.3.x.

Fixes #14458
Remove library/phpmailer
PHPMailer library is now a git submodule

A new 'mantis' branch was created from upstream's v5.2.6 tag, which will
be used to track MantisBT-specific changes to the library.

Fixes #14458
Revised README.libs after adding git submodules
Doc: updated URL to git web site
Doc: revise Contributors section

- Submodules initialization steps were added
- Many changes and improvements to sections 1.2 and 1.3
Doc: added note to explain checkout issues with submodules

Document error which can occur when switching to an older branch which
still has code from libraries in a subdirectory rather than a submodule.
Provide workaround, including also method to switch back.
Merge branch 'submodules' into 13x