Fix adding the same participant several times in a row

When a participant is selected in the select2 dropdown the "change"
event is triggered for the "Add participants" input; this sets the value
of the input and also adds the participant to the room. However, the
event is triggered only if the selected item is different than the
current value of the "Add participant" input. Due to this, the value has
to be cleared when it is set to make possible to add the same
participant several times in a row (for example, in different rooms).

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