Add a page to the homepage for Hack Brexit Open Source Accelerator 

1. [x] Create Open Source Accelerator page
2. [x] Link page to the homepage nav bar
3. [x] Add explanation of the Hack Brexit Open Source Accelerator, including links to T&Cs & Wikis 
4. [x] Direct visitors to Hack Brexit meet up group: https://www.meetup.com/Hack-Brexit/
5. [x] Include slack sign up link

@eddiejaoude - sounds good to you?
