WIP: Blocklist domain validation

[enbrnz]

Description

Verifying that domains entered into blocklist settings input form are actual domains. If not, showing error message with problem domain.

Motivation and Context

If an invalid domain is entered the blocklist feature might not work as expected.
Fixes:

guests/js/guests.js

Line 125 in 0d7d637

// FIXME: do validations here to make sure valid input is passed

Todo

Acceptance test

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.