Add modal to remind users to connect external accounts

[pjlast]

Closes #60616

Adds a modal that reminds users to connect external accounts for permissions syncing.

The modal does not show accounts that the user has already connected, and once dismissed, the modal won't be shown again unless a new auth provider is added.

The modal also only shows auth providers that would have an effect on permissions syncing. If no authorization is configured for any code host connections, then no modal will be shown.

See the attached Loom for a demo

https://www.loom.com/share/cb5f219bb9f3436fa1bcc1f70d603e67

Corresponding docs entry: https://github.com/sourcegraph/docs/pull/282

Test plan

Lots of manual testing and added a unit test for the resolver. Probably needs a few more tests

[sourcegraph-bot]

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