"Add contact" button should be added on Room Member list

[AmandineLP]

iOS should reproduce the behaviour of Android and have the same '+' button on the room member list as on the room list.
The text field at the top should be re-labeled 'Filter members'
An empty cell should be added at the bottom of the list so that the button doesn't overwrite the last item of the list (to be done in the room list too)

Clicking on the button should open the Default List screen as defined in #904

[giomfo]

Done, except #928.
Wait for validation

[jeangb]

Validated.