Add Ubuntu instructions to Postgres setup docs

Fixes bug 984436
Remove playdoh-lib submodules from gitmodules.

Though these directories have been removed, the submodule entries have
persisted.

Fixes bug 1042278
Merge pull request #2236 from bramwelt/remove-playdoh-lib-submodules

Fix bug 1042278 - Remove playdoh-lib submodules from gitmodules.
Merge pull request #2225 from bramwelt/fix-bug-984436-postgres-docs

Fixes bug 984436 - Add Ubuntu instructions to Postgres setup docs
bug 1042365 - some parsed_dumps don't have crash_info.crashing_thread