Tweak add-on signing in deployment to re-use previously signed XPI if version hasn't changed

[lmorchard]

Currently, all deployments of the site via the development branch attempt to sign the add-on build. This will fail if the add-on version hasn't been changed.

But, if nothing has changed in the add-on (i.e. no code, not the shared BG images), this is wasteful. Would be handy to tweak the deployment to just download and re-use an existing signed XPI from AMO. (We do this on Test Pilot, as an example.)

[lmorchard]

The only thing this issue prevents is continual automatic deployments to dev on commits to master. That would be nice to have, though I might say that having automatic master branch builds of the add-on signed could be seen as a negative by AMO folks.