**This PR creates a new core reusable component wraps a character counter around any input.**
The component accepts the arguments: `max` (the maximum character limit), `value` (the value of text to be monitored).
It can be used for example, like so:
```hbs
<CharCounter @max="50" @value={{this.charCounterContent}}>
<textarea
placeholder={{i18n "styleguide.sections.char_counter.placeholder"}}
{{on "input" (action (mut this.charCounterContent) value="target.value")}}
class="styleguide--char-counter"></textarea>
</CharCounter>
```
**This PR also:**
1. Applies this component to the chat plugins edit channel's *Edit Description** modal, thereby replacing the simple text area which provided no visual indication when text exceeded the max allowed characters.
2. Adds an example to the `/styleguide` route
