Use the display name in the avatar for a circle share

Before, the avatar for a circle share was generated using the
"share_with" field as the seed for "imageplaceholder". Due to this, when
the "share_with" field is set to the circle ID the character shown in
the avatar was just a random character instead of the first character of
the display name. Now the "share_with" is still used as the seed for the
colour, but the display name is used as the text of the avatar.

This adds support for "share_with" fields set to the circle ID while
being backwards compatible with "share_with" fields set to the circle
name.

Note that when "share_with" fields is set to the circle name the colour
of the avatar is different in the list of suggested sharees and in the
list of current sharees, but that also happened before these changes
(due to a different seed being used in each place).

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>