channel manager locking improvements

Several smaller improvements:

• When an Admin user is editing a channel, and then presses "Save and Close", an "Unlock channel" button is visible. The "Unlock channel" button should not be there if you are the owner of the lock.

• When an Admin user clicks the "Unlock channel" button, you now get the message "Are you sure you want to unlock this mount? All changes since the last publicized version will be lost."; the following is probably easier to understand for our users: "Are you sure you want to unlock this channel? All changes since the last publication will be lost."

• When you click "Discard all Changes", you now get the message "Are you sure you want to discard all changes in this channel?"; the following text is more explicit what will happen: "Are you sure you want to discard all changes in this channel? All changes since the last publication will be lost."

• In the channel manager overview, when viewing the channel in list mode, also display the "Locked by" column by default