Exchange "Link" for user-friendlier "Mix data" cf. Gus's comment

During testing of 4.4 -> 5.0 db upgrade the term "Link" have been deemed not to be too informative for the users. 


[See Line 3](https://docs.google.com/spreadsheet/ccc?key=0AoKiTAl8UOxndGt5bm5mOWl4M1lMc1NkQzVVRW5CZGc&usp=drive_web#gid=15) 

Also this PR fixes the inaccurate information about possibility of Copy between groups (this is not possible, only Move is possible).

To test, check [this page](https://www.openmicroscopy.org/site/support/omero5-staging/sysadmins/server-permissions.html) 


/cc @gusferguson @hflynn