Introduce WP_MAX_MEMORY_LIMIT constant for the high memory limit we set when...

Introduce WP_MAX_MEMORY_LIMIT constant for the high memory limit we set when image processing and unzipping.
Ensure it is always filterable by plugins as well as configurable in wp-config
Fixes #13847 props hakre


git-svn-id: https://core.svn.wordpress.org/trunk@17749 1a063a9b-81f0-0310-95a4-ce76da25c4cd