Add high severity support

[karlism]

Fixes https://github.com/camptocamp/grafana-prometheus-alertmanager-datasource/issues/134

[srindal]

Hi, we use: info, warning, error, critical - as severity levels. We have been using the angular version of your plugin for years, but this version seems to break our severity levels. Would it be possible to support both 'error' and 'high' as level 3? I understand that this can become really confusing I you have to support many different severitylevels, but I would be happy if you would and support for error = 3 in this PR. Thanks for building the plugin in the first place :-)