Sheet: use user-available-symbolic for send sms btn

Fixes https://source.puri.sm/Librem5/gnome-contacts/issues/28
2 jobs for send-sms-icon in 2 minutes and 41 seconds (queued for 25 seconds)
latest
Status Job ID Name Coverage
  Build Flatpak
passed #233659
flatpak
flatpak:master

00:02:39

 
  Build Arm Flatpak
failed #233660
ARM64
flatpak-arm64:master

00:00:02

 
Name Stage Failure
failed
flatpak-arm64:master Build Arm Flatpak
Running with gitlab-runner 12.4.0 (12.4.0)
on arm02 zx28gbiB
Using Docker executor with image jsparber/ci-arm64:arm64 ...
Pulling docker image gitlab-runner-helper:12.4.0 ...
ERROR: Job failed: Error response from daemon: pull access denied for gitlab-runner-helper, repository does not exist or may require 'docker login': denied: requested access to the resource is denied (executor_docker.go:186:1s)