Add ability to copy activity links in the editor sidebar

[2Pacalypse-]

This PR adds a button in the editor sidebar that allows content authors to easily copy links to other activities in the same repository. This is especially useful when content authors want to reference a different activity inside content elements.

The button is shown only on hover, and is shown for both editable and non-editable activities. This might be a bit of a controversial decision, but my thinking was that linking to non-editable activities might be useful outside Tailor, i.e. when content is published. Also, I feel like eventually, Tailor's editor itself might want to show something inside a non-editable activity, e.g. contents of that activity and/or options to add sub-activities to it.

[underscope]

Closing as this is being resolved on a fork