Localization strings:

- "Whitelist mode" -> "Limited federation mode"
- "Blacklist e-mail domain" -> "Block e-mail domain"
- "Whitelist domain" -> "Allow domain for federation"

...And so on

Environment variables (backwards-compatible):

- `WHITELIST_MODE` -> `LIMITED_FEDERATION_MODE`
- `EMAIL_DOMAIN_BLACKLIST` -> `EMAIL_DOMAIN_DENYLIST`
- `EMAIL_DOMAIN_WHITELIST` -> `EMAIL_DOMAIN_ALLOWLIST`

tootctl:

- `tootctl domains purge --whitelist-mode` -> `tootctl domains purge --limited-federation-mode`

Removed badly maintained and no longer relevant .env.production.sample file

* Remove trailing whitespace

* Use query methods instead of explicit .blank? checks

This adds the ability to filter user signup with a whitelist
instead of or in addition to a blacklist.

Fixes #817