Rename SpecialSetEntity to SpecialModifyTerm

As already done in the Api modules, the better name for the abstract base
class of SetLabel, SetDescription and SetAliases is of course
ModifyTerm. Also the messages have been changed to fit the new
naming convention.

Also update selenium tests that use these special pages

Patch set 3: use 'term' instead of 'modifylangattribute'

Change-Id: Ib25323bda95807ad7721182c76eb3dd179ac068f
Merge "Rename SpecialSetEntity to SpecialModifyTerm"