Implement all other store effects

[pcapriotti]

This is a followup to #1890, and part of step 2 of the polysemy refactoring series. It removes the MonadClient instance from Galley, and therefore makes all the DB-related code go through one of the store effects.

New effects:

• CustomBackendStore
• LegalHoldStore
• SearchVisibilityStore
• TeamFeatureStore
• TeamNotificationStore

Checklist

• The PR Title explains the impact of the change.
• The PR description provides context as to why the change should occur and what the code contributes to that effect. This could also be a link to a JIRA ticket or a Github issue, if there is one.
• changelog.d contains the following bits of information:
  • A file with the changelog entry in one or more suitable sub-sections. The sub-sections are marked by directories inside changelog.d.

[mdimjasevic]

Should grepping for "Cassandra" in the Galley/Data directory yield any import Cassandra lines? This is what I got by running ag "import Cassandra":

Scope.hs
22:import Cassandra hiding (Value)

Instances.hs
26:import Cassandra.CQL

ResultSet.hs
20:import Cassandra

Access.hs
20:import Cassandra

Queries.hs
23:import Cassandra as C hiding (Value)
24:import Cassandra.Util (Writetime)

Conversation.hs
35:import Cassandra

[pcapriotti]

I moved all the Cassandra-specific stuff out of Data. Unfortunately, some modules are still importing Cassandra modules directly, mostly for the paging stuff.