ZeroID-based domain names

The current proof-of-concept implementation:
https://github.com/geekless/ZeroNet/tree/IDName/plugins/IDName

- [x] Domains `.zeroid` are resolved with the ZeroID cert database.
- [ ] Cert sign verification is not yet implemented.
- [ ] Support for any ID provider can be added in the same way, if it exports a machine-readable database. But now it is only ZeroID, so no code for managing toplevel domains is present.

- [x] Subdomains `..zeroid` are resolved with a table from `.zeroid/content.json`
- [ ] No indication for long-running operations in UI yet. (Modification of the core Zeronet engine is probably needed.)

- [ ] Domain cache is silly yet.

Start with the linked IDName proof of concept in plugins/IDName and review the checklist against the current ZeroNet core engine. Investigate cert-sign verification, domain caching, provider support, and UI feedback for long-running operations; completion means the remaining unchecked behaviors are implemented and verified.

Written by the indexing model from the issue text.