* Added availability_template to Template Sensor platform
* Added to test for invalid values in availability_template
* Updated AVAILABILITY_TEMPLATE Rendering error
* Moved const to package Const.py
* Fix import order (pylint)
* Moved availability_template rendering to common loop
* Removed 'Magic' string
* Cleaned up const and compare lowercase result to 'true'
* Device is unavailbale if value template render fails
* Converted test ot Async and fixed states
* Comverted back to using boolean for _availability
* Fixed state check in test
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