Rework custom status context menu

This updates the custom status context menu to match the latest comps.  A single
button is used for setting / clearing, depending on what is appropriate.

The state logic is also changed to depend on events and storage from js-sdk for
the committed status message.  This makes it easy to distinguish the value being
edited from what's currently committed.

res/img/icons-checkmark.svg

@@ -1,17 +0,0 @@
-<?xml version="1.0" encoding="UTF-8"?>
-<svg width="20px" height="20px" viewBox="0 0 20 20" version="1.1" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink">
-    <!-- Generator: Sketch 52.5 (67469) - http://www.bohemiancoding.com/sketch -->
-    <title>Tick</title>
-    <desc>Created with Sketch.</desc>
-    <g id="Page-1" stroke="none" stroke-width="1" fill="none" fill-rule="evenodd">
-        <g id="Custom-Status-Copy" transform="translate(-529.000000, -917.000000)" fill-rule="nonzero">
-            <g id="Tick" transform="translate(530.000000, 918.000000)">
-                <circle id="Oval" stroke="#6AAC8C" fill="#75CFA6" cx="9" cy="9" r="9"></circle>
-                <g id="Glyph" transform="translate(8.949747, 7.949747) rotate(-45.000000) translate(-8.949747, -7.949747) translate(4.449747, 5.449747)" fill="#FFFFFF">
-                    <rect id="Rectangle" x="0" y="0" width="2" height="5"></rect>
-                    <rect id="Rectangle" x="0" y="3" width="9" height="2"></rect>
-                </g>
-            </g>
-        </g>
-    </g>
-</svg>