Add a clear action on the right of filter bar

Same behaviour as gitlab:

• display a small cross icon, a bit faded, on the far right of the bar
• when clicked, remove ALL filters
• only viewable when there are any filter set

added 1 deleted label

assigned to @erwanrouchet

mentioned in merge request !1038 (merged)

added 1 deleted label

closed

mentioned in commit 562b9473