impr(layout): adjust alignment of task dates and actors

Related to: CAM-3070
feat(variables): add checkbox for undefined variables in filter dialog

Related to: CAM-3058
feat(variables): display undefined variables when wanted

Related to: CAM-3058
impr(external form): make a call-for-action button to open the form

Related to: CAM-3064
impr(task list): change the style of assignee

And add a tooltip on the name in order to be able to read more or all of
the name.

Related to: CAM-3075
fix(forms): correct vertical alignment of checkboxes

Still, the size of that form element is changes depending on the browser and OS used
so it cannot be controlled precisely.
impr(styles): use variables for styling