Insights dashboard: drop dashboard configuration page in favor for in page locale selection

[functionzz]

In line with discussions made post-merge of #4189, we should remove the /insights/config page with its locale selector in favor of a solution that exists directly within /insights for more direct access. The UX can look similar to the "Request more projects" toggle found in Teams.

[functionzz]

I was thinking we can keep this page for locale selection for locale and project pretranslation quality as well.

[mathjazz]

I like the current locale/project selector in the pretranslation chart legend because it lets you instantly toggle lines on and off for easy comparison. Moving that functionality to a separate page would remove that capability.

I almost wish we could do something similar for the community health chart and tables, but it's probably an overkill.

[functionzz]

Yeah, I mean being able to select different locales - the current ones are fixed and cannot change. We can also add a new locale_pretranslation_locales and project_pretranslation_locales fields in the UserProfile model to customize the locales visible initially.