po: fix all po headers from git-gui to git-cola

This commit updates all po headers to the project named 'git-cola',
following the gettext po format.
* Basically, s/git-gui/git-cola/g
* Copyright declaration
    * Previous contributers of git-gui translation are still preserved
    * update copyright year according to git-cola's commit history  
      https://github.com/git-cola/git-cola/commits/master/po/*.po
* point Report-Msgid-Bugs-To entry to https://github.com/git-cola/git-cola/issues
* reset all language team mailing lists since those only applies for git-gui

Signed-off-by: Ｖ字龍(Vdragon) <pika1021@gmail.com>