Make exclusion reason available in statistic-picker

[karwosts]

Proposed change

Was thinking about ways to help with statistic selection and a lot of users struggle when their entities unexpectedly don't show up for picking. I was thinking maybe we can have a way to better surface why entities don't appear in specific pickers.

This change is such that everything appars normal originally (excluded entities are hidden), but when you click the "Why isn't my entity listed" button in the statistic picker, instead of linking to a website, we simply reload the picker but this time showing all entities and incompatible statistics, but then show the reason why we exclude them (entity is already selected, wrong device class, wrong unit, or entity doesn't have statistics). If you then select the "Why" button again it still goes to the original doc website.

Not entirely convinced this is necessary, but will put it out for opinions.

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New feature (thank you!)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

Example configuration

Additional information

• This PR fixes or closes issue: fixes #
• This PR is related to issue or discussion:
• Link to documentation pull request:

Checklist

• The code change is tested and works locally.
• There is no commented out code in this PR.
• Tests have been added to verify that the new code works.

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

• Documentation added/updated for www.home-assistant.io