Alert rule validation does not understand if/else constructs

[tanji]

Since the introduction of #4479, we are unable to validate a previously working alert message template containing the following:

{{ if eq .Level "OK" }}alert is under threshold{{ else }}alert is over threshold{{ end }}

This is a valid template construct as documented in https://golang.org/pkg/text/template/ and it works as intended, therefore it should be accepted as valid.

[ebb-tide]

fix validation logic for this case.