Add templates to trigger description

[litleleprikon]

It's often useful to add a Grafana dashboard link to a trigger description. But sometimes you have a templated dashboard. That is, you need to pass a specific variable value (e.g. host name) to open a dashboard that is relevant to the alert.

For example:
https://grafana.yourhost.com/some-dashboard?var-host=-METRICNAME-

We can add templating capabilities to our trigger descriptions to fill and render values at notification generation time.

Some things to consider:

1. What if a notification contains several events? How do we render the description? Do we pass an array of values to the template?
2. What do we render in web UI? A template? This can make links on the web unusable.
3. Which template language do we use? Which variables do we pass when rendering the template? Which templating capabilities do we allow so we don't compromise security?