client crashes when sources are reordered

[sssoleileraaa]

Description

Extracting issues in #906 that have an STR to make it easier for reviewers to test fixes.

STR

1. run NUM_SOURCES=10 make dev
2. run rm -rf ~/.securedrop_client && ./run.sh --sdc-home ~/.securedrop_client
3. select the first source in the source list
4. run NUM_SOURCES=10 make dev again to delete sources and add new sources and wait

Expected

For old sources to be deleted and new sources to show up.

Actual

The client crashes the the following traceback:

Traceback (most recent call last):
  File "/home/creviera/workspace/freedomofpress/securedrop-client/securedrop_client/gui/widgets.py", line 750, in __lt__
    my_ts = arrow.get(me.source.last_updated)
  File "/home/creviera/workspace/freedomofpress/securedrop-client/.venv/lib/python3.7/site-packages/sqlalchemy/orm/attributes.py", line 276, in __get__
    return self.impl.get(instance_state(instance), dict_)
  File "/home/creviera/workspace/freedomofpress/securedrop-client/.venv/lib/python3.7/site-packages/sqlalchemy/orm/attributes.py", line 677, in get
    value = state._load_expired(state, passive)
  File "/home/creviera/workspace/freedomofpress/securedrop-client/.venv/lib/python3.7/site-packages/sqlalchemy/orm/state.py", line 660, in _load_expired
    self.manager.deferred_scalar_loader(self, toload)
  File "/home/creviera/workspace/freedomofpress/securedrop-client/.venv/lib/python3.7/site-packages/sqlalchemy/orm/loading.py", line 985, in load_scalar_attributes
    raise orm_exc.ObjectDeletedError(state)
sqlalchemy.orm.exc.ObjectDeletedError: Instance '<Source at 0x7fae686423d0>' has been deleted, or its row is otherwise not present.