[auto-validator] models and admin for the core autovalidator service

[ppolewicz]

• manage subnets in admin
• trigger actions for validators to reinstall, dump logs etc from a selected number of validators
• The actions themselves will be implemented by someone else, but someone needs to do the basic database structure and configure admin
• • a celery task to update last_updated for those validators from the blockchain

[ppolewicz]

to be done by Fel

[ppolewicz]