Fixing user can't delete his own message

## Description:

lib/conversations.php[line 235]:
PreparePost funcation returns $author = user display name.

templates/part.conversation.php[line 19]:
if( USER_CONVERSATIONS_CAN_DELETE && ( 
OC_User::isAdminUser($uid) || $author == $uid ) ):

$author != $uid because the user display name is different than his username.

Example [add user in OC]:
username: shachar fullname:Shachar Ochyon