Adjust monitoring graphs to support widgets in EE

In order to facilitate gitlab-ee#5158 with minimal CE/EE conflicts, I'm adding a <slot> within graph.vue so that the EE-specific changes needed to support graph alerts will be limited to dashboard.vue.

Although the "alert" widget is not included in this MR, this is how it would look and behave when something is added to the new slot in the graph header:

This utilizes flexbox, and I've tested that it behaves appropriately when the title content is long enough to collide with the button:

And it does not break when graphs are side-by-side: