Create basic file structure for Context.IO access

* Create module for Sellevate
* Create submodule for Context.IO
* Load Context.IO API keys
context.io auth: get initial version working

API call to Context.IO that starts the OAuth pipeline flow that allows
us to get access to a user's mailbox in our Context.IO system.
Rename Sellevate to Fancy Bear
fancy_bear.rb: remove test code

Delete lines to test the auth functionality.
auth.rb: fix authentication method

* Fix incorrect variable name
* Rename authenticate method because that's not quite what it's doing.
  It's more like starting the auth process.
fancy_bear/: use relative requires

I can't import fancy_bear in app.rb without using relative requires
here.
layout.erb: create a basic layout file

Not sure if Jonathan already created one. Using this so I can test
things out.
app.rb: add Context.IO oauth routes/actions

* Require the FancyBear module
* Add a couple new methods to support authentication via Context.IO
Add auth_callback.erb

To support the action I created in app.rb previously.
index.erb: update with link to authorise Gmail

* Add an image for laughs
* Add a link that starts the Gmail OAuth pipeline through Context.IO
Merge branch 'feature/contextio-auth' into develop