Migrate to the new PI and co-proposer modals

[simonfernandes]

The ESS have developed two new modals for adding a PI and co-proposer. This issue contains the steps we need to take to migrate to them on prod.

Context

The co-proposer modal was implemented in #975 and #970 and adapted for STFC use in #1161. The PI modal was implemented in #1176. Both modals will only show if EMAIL_INVITE_LEGACY feature is disabled.

We very briefly used the co-proposer modal on prod but toggled it off it as a precaution due to #1470. The co-proposer modal will need to be retested as it has been refactored in the PI modal work.

Migration steps

• Review the PI modal PR to ensure it is completely hidden behind a feature flag
• (Re)test both modals on dev once the PI modal PR is merged
• Check @Cat-Lucifer is happy with the modals on dev
• Make any necessary fixes/tweaks
• Release the PI modal to prod
• Toggle both modals to show, at a suitable time
• Give green light to ESS for EMAIL_INVITE_LEGACY feature removal

[simonfernandes]

Released to prod and toggled on as of Friday 12th Dec. Made ESS aware