event pane: make host and service configurable links

As suggested in my comment in #76.

To avoid having to define the URL in each and every view, I decided to make these global parameters of the dash. The downside is that there are ruby variables containing javascript templates, which are passed over to javascript in global variables, as part of a ruby template... A bit of a mess !

If you don't like this, I can confine it better by making these URLs configurable on a per-view basis (at the expense of having to configure them each time).