This is another pure refactor, to prepare for adding direct WSL support.

###  Todo

* [x] Represent `paths` in the same way for all workspaces, instead of
having a completely separate SSH representation
* [x] Adjust sqlite tables
    * [x] `ssh_projects` -> `ssh_connections` (drop paths)
    * [x] `workspaces.local_paths` -> `paths`
    * [x] remove duplicate path columns on `workspaces`
* [x] Add migrations for backward-compatibility

Release Notes:

- N/A

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Co-authored-by: Mikayla Maki <mikayla.c.maki@gmail.com>