Externalize slash mechanism

[3esmit]

Fixes #105

With this PR we make rules for slash mechanism in a separate contract, which can be more easily maintained, audited and replaced.

This change will allow Status to replace the rules for "Invalid/Reserved Usernames" without changing the ENS Usernames address.

TODO LIST:

• [x] Separate Slash Mechanism to a dedicated contract; (a5f7f8e)
• [x] Allow controller to update the Slash Mechanism Address; (612c063)
• [x] Returns funds to registrant when usernames does not offend former slash mechanism but offends the updated slash mechanism; (6a2acce)
• [x] Allow instant release of names if Slash Mechanism is changed. (31a1a9d)

[3esmit]

• Rebased to master (now in Embark 6, solc 0.5.11);
• Changes requested by @rajeevgopalakrishna were fixed (and the conversations closed);
• All tests are passing with the external slash mechanism;
• Crytic is working https://crytic.io/status-im/ens-usernames/15/security_checks.

I'm waiting for:

• @rajeevgopalakrishna to close the conversations that were opened and answered, or new comments if not agree on response.
• "Request changes" or "approval" from @0kok0, @rajeevgopalakrishna and @corpetty.