Dstack-tasks

- Add new `db` task that optionally supports replication
- Add `build-superset` task to make local dev easier with docker & nginx
- Add `send-email` and `send-alert` tasks.
`send-alert` supports Slack, Telegram and Microsoft Teams
- Absorb `dstack-server` project to create servers more reliably and
configure ssh config locally
- Add `dstack-bot` install task
- Add `docker-machine` wrapper
- Update base `do` command with `sudo` support
- Update `release-code` to support webpack based static build system
- Removed deprecated tasks and dependencies and make code more generic

Transition to V2

dstack_tasks V1 was build on a Python3 port of `fabric`
V2 is a rewrite build on `invoke` with support for `fabric2`.

This version is Python 3.6 only as it makes extensive use of f-strings,
but since it's meant to be used as a standalone tool, it does not matter
which version of Python your tool uses.

The tools does not yet take advantage of annotations, as there is an
open bug related to inspect.signature in invoke.

See: https://github.com/pyinvoke/invoke/pull/373