Make persons config page translatable

[bramkragten]

All text in the Home Assistant UI should be translated.

The Persons config panel is not translated yet.

• Create new keys in src/translations/en.json with the text that is currently hard coded in the source.
  Try to follow the same logic that is present, and give every section in Home Assistant it's own object.
  If you would place the text in

  "ui": {
    "panel": {
      "example": {
        "text": "Here goes the text",
      },
    },
  },

your key would be: "ui.panel.example.text

Polymer Elements need a mixin: LocalizeMixin(PolymerElement), Lit Elements work without a mixin.

• Replace the text in the code with:
  • Lit element: ${this.hass.localize("ui.panel.example.text")}
  • Polymer: [[localize('ui.panel.history.showing_entries')]]

(This is part of #3429)