Add link asset fields to "Edit Release" form

What does this MR do?

Adds link assets fields to the "Edit Release" form as described in #9427 (closed).

Validation

This MR does not implement any form validation, although this will be a part of this feature before it is released.

Form validation will be implemented in a separate MR in order to keep this MR small.

Feature flag

This feature is hidden behind a release_asset_link_editing feature flag which is currently disabled by default.

Screenshot

Responsive behavior

Screen recording

https://drive.google.com/open?id=19dzz-WmzH0COIdYS-gM0OQxl59f710hE