master<-trunk

[hisahi]

Short description

• 898: Add option to specify tag color as hex

• 956: Checklists can have subchecklists

• 1026: Do not import links from other course

• 1033: Confirm if the student wants to remove their registration

• 1035: When sending mass email, remove title "You've received a message from Labtool"

• 1060: Internal server error when copying and saving checklists from another week or course

• 1061: JSON isn't updated automatically when modify checklists and click save

  Extensive testing is recommended for #956

DoD

I have:

• [x] added actual code.
• [x] produced clean code that passes ESLint.
• [x] code that actually does what it should.
• [x] documented the code or added documentation to the wiki.
• [x] added or modified test(s).
• [x] test(s) that actually pass.
• [x] works in trunk branch