Improve server validators

[promaty]

See Brett's comments:

• Server: Email verification tokens and validators #153 (comment)
• Server: handle emails with multiple addresses #157 (comment)

Each validation should be a different instance so wouldn't need to rely on resetErrors function which can be a bit brittle.

[promaty]

Example of different implementation in ipfs-pinner:
https://github.com/aragonone/ipfs-pinner/blob/development/packages/server/src/helpers/files-validator.ts