Add Tutorials Section

[ceolson01]

Description

New feature. This PR aims to add a tutorials section powered by content from Colony's new Tutorials repo!

🌮 🌮 🌮

⚠️ Note: The sidebar will not be implemented at this time, as it's unknown as to what should go into it 🤷‍♂️ Additionally, the dev portal is actually going to be launched without the tutorials. Hiding the links to the tutorials section, and disabling the tutorial page will be handled via #44.

Notes for Reviewer

1. To run this project, you'll also need to clone the tutorials repo (linked above) and check out the following branch before running yarn dev: https://github.com/JoinColony/colonyTutorials/pull/1
2. The Staging Site currently contains work from both this PR and Dev Portal - Designer Edits #44.

Changes

• Add TutorialPage component for handling single tutorial page instances.
• Add Tutorials page for the tutorials list.
• Add gatsby-transform-md-tutorials plugin for transforming tutorials markdown content & providing graphql query nodes to query for said content.
• Add tutorials info to gatsby-config.
• Update transform plugins to only transform data from particular projects (defined via gatsby-config).
• Add some new Button themes.
• Update background pattern image to new image which has reduced opacity (ends up being removed via Dev Portal - Designer Edits #44).
• New gitter icon (with transparent negative space).

Closes #22
Contributes to #33

[rdig]

Good to go 🙂