## Description
Added custom icons that don't meet the requirements of Simple Icons
(especially because they are full-color).
The Airtable and Discourse icons are an upgrade to the single-color
icons in Simple Icons, the others are not available yet at all.
## Type of Change
<!--- Put an `x` in all the boxes that apply: -->
- [x] 🖼️ New icon
- [ ] ✨ New feature (non-breaking change which adds functionality)
- [ ] 🛠️ Bug fix (non-breaking change which fixes an issue)
- [ ] ❌ Breaking change (fix or feature that would cause existing
functionality to change)
- [ ] 🧹 Code refactor
- [ ] ✅ Build configuration change
- [ ] 📝 Documentation
- [ ] 🗑️ Chore
cf55c48b4c
