Use the stow wrist service

[hello-amal]

Currently, the web app stows the wrist by moving to hardcoded joint positions. However, the right stow joint configurations depend on the tool (e.g., Stretch 3 gripper is longer than Stretch 2 gripper) and possibly other factors. stretch_ros2's service to stow the robot already accounts for these factors. Thus, to centralize code and gain more generalizable functionality, the web app should stow the wrist by calling that service.

Blocked by stretch_ros2#143.

Related to #60 .