[IMP] mail: channel seen icon indication ('v' and 'vv')

This commit adds the seen icon next to a message for all channels
except livechat.

It tracks received and seen messages for all members of the channel.
Icon is either 1 or 2 check(s) ('v' or 'vv').

Those are the different states of this 'seen' icon:

  ICON:                           DESCRIPTION:

  no check .................. no one has received the message.
  1 check ................... some members have received the
                              message, no one has seen it.
  2 checks (greyed-out) ..... some members have seen the message,
                              some or everyone has received it
  2 checks (green) .......... all members have seen the message

This icon is not shown for messages preceding the last message seen
by everyone, in order to reduce visual noise.

Task-ID 1881001

0 files changed, 0 insertions, 0 deletions