changing expires for modules and plugins from 10 years to 1 month, as discussed on the mailing list

git-svn-id: file:///var/local/code.tiki.org/tikisvn/branches/12.x@48125 b456876b-0849-0410-b77d-98878d47e9d5
[MOD] Activating ExpiresDefault by default in _htaccess because the vast majority of sites want this. However, changed default value from 6 months to 1 month. Also added a note for developers.

git-svn-id: file:///var/local/code.tiki.org/tikisvn/branches/12.x@48127 b456876b-0849-0410-b77d-98878d47e9d5