Move the styles used by `ThreadSettingsModal` in `style.css` to `thread-settings-moda.css` (if they aren't used elsewhere in repo, in which case they were copied) and "consume" `thread-settings-modal.css` in `ThreadSettingsModal`.
This already gets us closer to where we want to be.
Before:
{F28042}
After:
{F28043}
Note: The `current-tab`, `delete-tab` and `modal-radio` selectors are neither in `style.css` nor in `user-settings-modal.css` (the file that `thread-settings-modal.css` was copied from). Will handle styling of these selectors in later diff.
Next diffs:
1. Remove extraneous selectors from `thread-settings-modal.css` that aren't used in the `ThreadSettingsModal` component.
2. Fill in styles that are "used" by `ThreadSettingsModal` that don't actually exist anywhere in the repo (eg `current-tab`). Probably by looking through the git history to see what `current-tab` **used** to be.
3. Style the `ThreadSettingsModal` component so it looks decent.
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Depends on D3589