DEV: group_list site settings should store IDs instead of group names

[romanrizzi]

Why do we need this?

Identifying groups by name is not reliable enough when you want to use defaults since they could have been renamed. IDs are a better alternative because they are unique.

After some discussion here, we decided that we should change core to simplify the plugin logic and use defaults safely.

[discoursebot]

You've signed the CLA, romanrizzi. Thank you! This pull request is ready for review.

[eviltrout]

My first question is often: is there a context for this change? It always helps to tie it back to a post on one of our forums in case someone in the future wants to know why we did this.

Secondly, as you noticed in your plugin follow ups this is a breaking change. Did you consider adding a new site setting type, for example, group_id.

In that case we can use group ids for new settings without breaking old ones in plugins.

[nlalonde]

I'm guessing the reason for this is because group names are visible in client code.

[romanrizzi]

I updated the plugin description with some context. Only discourse-adplugin and discourse-assign use this setting type. Maintaining two different setting types will be confusing since we should always use IDs to reference groups.