When a port is specified for localhost, it should still be accepted as valid

[MariagraziaAlastra]

Reviewer: @dharb

CC: @andrey-p

Description:

Following up from #199, this also allows to whitelist the localhost when a port is specified (currently, strings like 'localhost:8080' are not considered valid).

Steps to test this PR:

1. Build and reload extension
2. Go to the options page
3. Try whitelisting 'localhost', 'localhost:8080' (or use any port) - both should be valid
4. Things like 'localhost:asdlajshdkja' should still be invalid
5. Try whitelisting other domains: they should get validated as usual
6. After whitelisting a domain, navigate to it and make sure it's actually whitelisted
7. Play around with adding and removing domains using the toggle and the options page to make sure functionality is intact

Automated tests:

• Unit tests
• Integration tests

Reviewer Checklist:

• Ensure the PR solves the problem
• Review every line of code
• Ensure the PR does no harm by testing the changes thoroughly
• Get help if you're uncomfortable with any of the above!
• Determine if there are any quick wins that improve the implementation

PR Author Checklist:

• Get advice or leverage existing code
• Agree on technical approach with reviewer (if the changes are nuanced)
• Ensure that there is a testing strategy (and documented non-automated tests)
• Ensure there is a documented monitoring strategy (if necessary)
• Consider systems implications

[dharb]

Thanks for taking care of this, looks great!