Removable labels from filtered issuables label bar

When filtering by labels, a remove button appears next to each label. This then removes that label & refreshes the issuable filter form

![Screen_Shot_2016-05-17_at_12.07.47](/uploads/b21e86247aa1bbfd347bef1eb21f1562/Screen_Shot_2016-05-17_at_12.07.47.png)

Closes #15474

See merge request !4178