An ability to 'favourite' or 'follow' a checklist

[wai2k]

Have to careful what this really means.

Has to be distinct from 'cloning a checklist'

We may be to choose the terms carefully. 'Favouriting' a checklist may mean choosing to favourite a particular version of a checklist.

Whereas 'following' may mean following a particular thread of branch of a checklist.

Alternatively, we can only have once 'favourite' or 'follow' function but have a toggle for users to decide if they want to receive automatic updates of a checklist.

[ntoll]

Similar to #44