Native integrations: add opt-in experience

Created by: lguychard

Currently, when installed by the site admin, native integrations will be installed by default for all users.

Some customers have expressed interest in having the following experience:

• Default is still opt-out
• Site admin can decide to change that default
• Users would then have to individually opt-in

Customers having expressed interest in this:

• https://app.hubspot.com/contacts/2762526/company/464956351 (Phabricator native integration)
• https://app.hubspot.com/contacts/2762526/company/407948923 (Bitbucket native integration)