Rewrite admin documentation

Closes #1333 (closed)

Closes #1195 (closed)

Closes #1386 (closed)

Closes #1502 (closed)

Closes #1537 (closed)

Now that the install guides and migration guide are complete, it's time to start moving the rest of the admin documentation in.

Preview link

Tasks

• Django admin
• manage.py tasks
• Change instance URL
• Upgrade guide
• Instance configuration
• Import music from server
• S3 storage guide
• Backup guide
• Migration to new server
• Uninstall Funkwhale
• LDAP configuration
• Message Rewrite Facility
• Troubleshooting
• Add redirects