[noypiscripter]

These position the Subscribe button to the rightmost spot without the big gap at the bottom. These should work on both desktop and mobile.

.conversation-content-widget .conversation-controls {
    position: relative;
    bottom: -22px;
    z-index: 1;
    margin: -26px 0 0;
    right: 0;
    float: right;
    padding: 0;
}
.canvas-layout-container .canvas-widget.page-title-widget {
    margin-bottom: 0;
}

I think that looks better than putting the Subscribe button right next to Posts and Latest Activity tabs.