Request to add Priest's Mind Spike rune for Season of Discovery.

It stacks the Mind Spike buff up to 3 times. The buff improves next cast of Mind Blast.

The effect should display a spell alert and a glowing button:

• display a purple-ish Fingers of Frost spell alert
  • the effect should start small at 1 stack, no pulse, and end up big and pulse at 3 stacks
• the Mind Blast button should glow
  • glowing at 3 stacks for sure, not sure about 1-2 stacks

Implemented in #223.