Fallback to the term slug if a label does not exist in list-table

Relates to #214
Merge branch 'develop' into issue-deactivated-connectors

Conflicts:
	includes/list-table.php
Add tax singular label to action links
Merge pull request #301 from x-team/issue-deactivated-connectors

Fallback to the term slug if a label does not exist in list-table