Implement a mark-as-merged action for shipitscript (bug 1890753)#1312

Eijebong wants to merge 1 commit intomozilla-releng:mastermozilla-releng/scriptworker-scripts:masterfrom

Eijebong:shipit-mark-as-mergedEijebong/scriptworker-scripts:shipit-mark-as-mergedCopy head branch name to clipboard