Template Sensor add device_class support

[OttoWinter]

Description:

Sensor now can have device classes too (see #14010) and AFAIK they're now the recommended to use instead of the old icon: configuration option.

Related issue (if applicable): fixes #

Pull request in home-assistant.github.io with documentation (if applicable): home-assistant/home-assistant.io#5220

Example entry for configuration.yaml (if applicable):

sensor:
  - platform: template
    battery:
      value_template: "{{ states.device_tracker.iphone.attributes.battery }}"
      device_class: battery

Checklist:

• The code change is tested and works locally.
• Local tests pass with tox. Your PR cannot be merged unless tests pass

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

• Documentation added/updated in home-assistant.github.io

If the code does not interact with devices:

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

[houndci-bot]

line too long (80 > 79 characters)

[pvizeli]

Need tests

[OttoWinter]

Tests have been added :)

Concerns addressed