RF: linked pages

[vleong99]

• Moved linked pages to checks
• Added tests for logic-handling and for the linked pages handler. Tests use a mock client instead of making an actual GET request to avoid flakiness
  • NOTE: The resolveURL function parses hrefs in the same way a browser would, which can lead to some unintuitive outcomes. I have left a comment by that function with further explanation and added some test cases for illustration
• Modified the sortURLsByFrequency function to ensure that results are deterministic
• Modified the walkDom function to ensure that "www" subdomains are disregarded when determining if a link is internal (previously, "www.google.com" would be considered an external link to "google.com", which seems like incorrect behaviour)

[codecov]

Codecov Report

Attention: Patch coverage is 81.25000% with 15 lines in your changes missing coverage. Please review.

Flag	Coverage Δ
unittests	59.43% <81.25%> (+1.70%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
handlers/linked_pages.go	100.00% <100.00%> (+33.33%)	⬆️
server/server.go	84.21% <100.00%> (ø)
checks/checks.go	0.00% <0.00%> (ø)
checks/linked_pages.go	86.88% <86.88%> (ø)

[Lissy93]

Thanks, this looks awesome 🙌