Include linkchecker tests

[busykoala]

🚧 DO NOT MERGE YET 🚧

Close #20

Test being added:

• Are external links detected?
• Are broken relations found?
• Does the excel report generator work correctly?
• Does the mail sender correctly send an email including attachment?

Additional Changes needed to implement tests:

• Adapt excel generator (remove function calls in module outside class). (Close #8 )

[busykoala]

I merged this branch too early. It need additional tests and review.