Regression since the mass-replace in 34cdaade0.

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/2054

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/1939

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/1749

We were applying the low-memory system patch to webpack.config.js
on systems with < 4GB memory.

From now on, we also let people force-enable patching by toggling the
`matrix_client_element_container_image_self_build_low_memory_system_patch_enabled`
variable.

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/1357

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/1357

Should have been done in 63f5a88fa0.

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/1070

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/716

This patch makes us use more fully-qualified container image names
(either prefixed with docker.io/ or with localhost/).

The latter happens when self-building is enabled.

We've recently had issues where if an image was removed manually
and the service was restarted (making `docker run` fetch it from Docker Hub, etc.),
we'd end up with a pulled image, even though we're aiming for a self-built one.
Re-running the playbook would then not do a rebuild, because:
- the image with that name already exists (even though it's something
else)
- we sometimes had conditional logic where we'd build only if the git
repo changed

By explicitly changing the name of the images (prefixing with localhost/),
we avoid such confusion and the possibility that we'd automatically pul something
which is not what we expect.

Also, I've removed that condition where building would happen on git
changes only. We now always build (unless an image with that name
already exists). We just force-build when the git repo changes.

Fixes #586 (Github Issue)