Adds Stale Probot for issues

[frenck]

Description:

This PR adds the Stale Probot to this repository.

We have tons of issues open and from time to time we do manual runs using the @balloobbot. However, this relies on manual interaction from maintainers.

The Stale Probot can do this for us continuously.

It has been configured to ignore:

• Pull requests
• Issues placed on a project board
• Issues placed in a milestone
• Issues with an assignee
• Issues labeled as under investigation

The current configuration ONLY handles issues! PR's are untouched.

Current configuration leaves a message on an issue after 90 days of inactivity and adds the stale label. I took the @balloobbot message and extended it a little:

There hasn't been any activity on this issue recently. Due to the high number
of incoming GitHub notifications, we have to clean some of the old issues,
as many of them have already been resolved with the latest updates.

Please make sure to update to the latest Home Assistant version and check
if that solves the issue. Let us know if that works for you by adding a
comment 👍

This now has been marked as stale and will be closed if no further
activity occurs. Thank you for your contributions.

After another 7 days of inactivity, it closes the issue without further comments.

If not already, the Stale Probot needs to be installed to this repository:
https://github.com/apps/stale

Related issue (if applicable): n/a

Pull request with documentation for home-assistant.io (if applicable): n/a

Example entry for configuration.yaml (if applicable): n/a

Checklist:

• The code change is tested and works locally.
• Local tests pass with tox. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.
• I have followed the development checklist

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated in home-assistant.io

If the code communicates with devices, web services, or third-party tools:

• The manifest file has all fields filled out correctly. Update and include derived files by running python3 -m script.hassfest.
• New or updated dependencies have been added to requirements_all.txt by running python3 -m script.gen_requirements_all.
• Untested files have been added to .coveragerc.

If the code does not interact with devices:

• Tests have been added to verify that the new code works.