restructure links and link types

[wavehassman]

Changes

• Moves dashboard placement (guest home page, new member dashboard, onboarding dashboard) off the shared link type and onto individual links, so the same link type can now be reused across a project link and any number of dashboards instead of being tied to one placement
• Also cleans up a handful of related rough edges: a database transaction now protects the useful-links save so a failed request can't wipe out an org's existing links, the migration backfill makes sure old data doesn't end up assigned to more than one dashboard, and a couple of frontend components were fixed so save failures actually show an error instead of silently closing or getting stuck on a loading spinner

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[staysgt]

is there a reason that you chose to do this with the three separate fields instead of an enum field for a link's page? In terms of clicking around on the frontend it doesn't seem like theres a way to have a link set to more than one page, and I think an enum would make some of the logic more straightforward