Support for built-in templates

We should ship built-in Integrated Alerting Templates with PMM Server. To keep them closer to pmm-managed code that will validate them during running and on CI, we should keep them in pmm-managed repository, similar to how we do with Check Service. Unlike Check Server, we don't build a Docker image; instead, we should use go-bindata to include them into pmm-managed binary. We should not use pmm-server-packaging or pmm-update for that.

We also should start managing directories with pmm-managed, not pmm-update, to support Docker-way (swapping image) PMM updates. See https://github.com/percona/pmm-update/pull/112.

• Generalize and extract Alertmanager.Service.createDataDir method into utils/dir package. It should create or update a directory with given permissions (both chmod and chown). Use it inside Alertmanager.Service.

• Use that function to create new directories on pmm-managed startup: /srv/ia/templates, /etc/ia/rules.

• Use go-bindata and make gen to convert IA template files to Go code and include it into the build.

• Use those built-in templates in the template collection code from https://perconadev.atlassian.net/browse/PMM-6873#icft=PMM-6873. Change /tmp/ia1 to /srv/ia/templates and remove /tmp/ia2. Update tests.

• Write a unit test that ensures that all collected templates are valid.