GitHub Integration v1

[reichert621]

Description

This PR will:

• Set up the github_authorization table
• Set up a GitHub API client
• Handle both OAuth and App installs
• Set up the UX for authorizing Github
• Set up a webhook URL for handling events
• Set up the issues table
• Support linking a Papercups issue to a GitHub issue
• Support updating a Papercups issue's state based on the GitHub issue state

TODOs for followup PRs:

• User can send emails to customers from customer details page
• User can select main GitHub repo to link to Papercups (since GitHub supports multiple by default)
• User can create new issues in GitHub directly from Papercups
• User can search GitHub issues when linking Papercups issue to GitHub (rather than pasting in GitHub URL)
• User can perform more actions from Papercups Issues page (e.g. update state)
• User can edit Papercups Issues

Issue

#745
#749

Screenshots

Authorization flow:

Create new Papercups issue:

Link issue to existing customer:

Link customer to existing issue:

Sync state with GitHub:

Checklist

• Everything passes when running mix test
• Ran mix format
• No frontend compilation warnings