Alerting: update panel alert tab

[domasx2]

What this PR does / why we need it:

• Reworked pamel alerts tab for unified alerting. Shows alert rules that match the panel either by dashboard & panel id annotations or by panel URL annotation. Has a button that takes you to rule form pre-filled with queries, rule name and folder from the panel
• Possible to provide default values to rule form via URL defaultValues query param. Expects JSON with form values
• Possible to specify where the rule form should return to on "Cancel" or "Save and exit" with returnTo query param.
• More suggested annotation options
• Alert threshold not shown on the graph for unified alerting. Currently don't have an easy way to calculate this threshold. Will liekly be revisited later
• Condition field in rule form automatically selects the last expression if one exists
• Some annotations in rule/alert details show "readable" name with tooltip for the real key. This is somewhat odd, might need more thinking about in the future. Real key is important as that's what you'd use in a template

TODO:

• remove the dasboard_url and panel_url annotations, these are to be generated at runtime. Not quite sure how to link existing rules to panels though.. perhaps that's not needed

panel.alerts.mp4

Which issue(s) this PR fixes:

Fixes https://github.com/grafana/alerting-squad/issues/9
Fixes https://github.com/grafana/alerting-squad/issues/141
Fixes https://github.com/grafana/alerting-squad/issues/110

Special notes for your reviewer:

I went with an odd way to get the number of rules into panel alerting tab. Better ideas are welcome!

Does anything else need to be modified?

[nathanrodman]

Great work! It's pretty seemless going from panel to alert rule creation, and the navigation back to the panel from the rule list is awesome.

[bfmatei]

Just some super minor nitpicks