Improve handling of SRE names

[JimMadge]

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⤴️ Summary

Improve how SRE names are handled,

• Use a single function for sanitising the name from user input across the code
• Remove unused methods/lines for adding/removing SREs (this is done in the config)

🌂 Related issues

Closes #1687

🔬 Tests

[JimMadge]

@craddm @jemrobinson could we get reviews on this? It impacts #1716

[jemrobinson]

Mostly looks fine, but we should think about how to remove deleted SREs from the config file if there's no remove_sre function.

[jemrobinson]

LGTM