Add ability to pull down Thunderbird extensions

Extensions should be put in netlogon/domain_information/thunderbird,
and must have the extension '.xpi'.
Remove Thunderbird LDAP directory settings

Contacts are now handled through SOGo
Add ability to pull down architecture-specific Thunderbird extensions

Global extensions should be in
netlogon/domain_information/thunderbird/extensions, while Linux-specific
extensions go in netlogon/domain_information/thunderbird/extensions/linux-ARCH.
Architecture name can be found using 'uname -i'. For example, Linux on x86_64
would use the folder extensions/linux-x86_64.
Fix errors in Thunderbird extension code; remove need for temporary directory
Make unzip output quiet, fix error removing files
Fix extension ID extraction using xmlstarlet to support attribute em:id
Add mail reader to application menu, and remove some excludes
Fix nested symlink error in Thunderbird extension installation
Merge branch 'sogo'