Add Alertmanager config and templates in Helm chart (#188)

* Add Alertmanager config and templates in Helm chart

- Add secret resource, embedding raw and templated alertmanager files
- Expose alertmanager templates values as helm chart values
- Remove all Mimir related conditions
- Split template into url and notification templates
- Drop template directive, dynamically set by the operator
- Escape template in template
- Re-use slack actions

* Add alertmanagerURL

* rename alertmanager to alerting in values.yaml

* Use proxy_from_environment in Alertmanager

* remove proxyURL from values

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Co-authored-by: Taylor Bot <[email protected]>
Co-authored-by: Quentin Bisson <[email protected]>