usability: Enlarge 'Send' button touch surface
usability: Enlarge 'Send' button touch surface

Thanks to the previous commit, we can now style the 'Send' button
with 'padding' instead of 'margin'.

Visually nothing changes, but now the touch area extends 8 pixels
in each direction.

PS. Padding is part of the component, margin is outside of it.

vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile