fix: show deletion mutations in sync status UI

[scolsen]

Before, we cascaded deletions on the foreign keys for feature/observation mutations, meaning all mutations for a feature/observation were wiped from the DB, eradicating the user's edit/sync history.

This change alters that behavior so that we keep mutations around even after a feature is deleted. Additionally, it removes a redundant call to deleteFeature when handling mutations (these are handled server side and should not be finalized until deleted on the server--the local project receives this update via syncFeatures).

fixes #1059

[scolsen]

@gino-m ptal! this will fix #1059 but gives us a new problem to solve: It persists mutations longer than the features/observations they belong to. We'll have to determine a good strategy for deleting these over time.

[gino-m]

Hey @scolsen do you think it's possible to merge with HEAD? If they've diverged too much should we close this one? :cry:

[gino-m]

@scolsen Is this PR still active? If not can we close it?

[gino-m]

Closing for now due to inactivity. Please reopen if necessary!