Add a link to generate a new bearer token

https://www.pivotaltracker.com/story/show/69285272

[#69285272]

- The link warns the user if there is already a bearer token set
Use $ instead of jQuery

- Style thing really, I think `$` is the idiomatic and short reference for `jQuery`
Take `CHARS` out of global scope
Reference jQuery stored vars with $, change order

- $someElement should hopefully make it clear that this element is now a jQuery object, accessed via a jQuery selector
- Adding the element to the DOM after it's been defined, hopefully making it harder to miss
Merge pull request #124 from alphagov/autogenerate-bearer-token

Add a link to generate a new bearer token