Fix "Can't determine which FROM clause to join from" SQLAlchemy error (#38)

Something changed in SQLAlchemy 1.3 after we upgraded (maybe in 5b02472?) 
which caused the graph and matrix queries to start erroring, showing something like:

    > InvalidRequestError("Can't determine which FROM clause to join from, there are 
      multiple FROMS which can join to this entity. Please use the .select_from() method
      to establish an explicit left side, as well as providing an explcit ON clause if not
      present already to help resolve the ambiguity.")

This fixes the errors by explicitly specifying which table to use as the FROM query.
From what I understand we only have this error if there are multiple joins in the same query.

The query in load_geomean_data also seemed to be missing a join on the samples table.
I guess SQLAlchemy must have previously inferred the join or something? Since the 
relations go run -> sample -> test.

This affected both the sqlite and postgres backends, and I've tested this fixes the 
error with both.

Author: lukel97

lnt/server/ui/views.py

@@ -881,6 +881,7 @@ def load_graph_data(plot_parameter, show_failures, limit, xaxis_date, revision_c
     # we want to load. Actually, we should just make this a single query.
     values = session.query(plot_parameter.field.column, ts.Order,
                            ts.Run.start_time, ts.Run.id) \
+                    .select_from(ts.Sample) \
                     .join(ts.Run).join(ts.Order) \
                     .filter(ts.Run.machine_id == plot_parameter.machine.id) \
                     .filter(ts.Sample.test == plot_parameter.test) \
@@ -924,6 +925,7 @@ def load_geomean_data(field, machine, limit, xaxis_date, revision_cache=None):
     values = session.query(sqlalchemy.sql.func.min(field.column),
                            ts.Order,
                            sqlalchemy.sql.func.min(ts.Run.start_time)) \
+                    .select_from(ts.Sample) \
                     .join(ts.Run).join(ts.Order).join(ts.Test) \
                     .filter(ts.Run.machine_id == machine.id) \
                     .filter(field.column.isnot(None)) \
@@ -1105,6 +1107,7 @@ def v4_graph():
             q_baseline = session.query(req.field.column,
                                        ts.Order.llvm_project_revision,
                                        ts.Run.start_time, ts.Machine.name) \
+                         .select_from(ts.Sample) \
                          .join(ts.Run).join(ts.Order).join(ts.Machine) \
                          .filter(ts.Run.id == baseline.id) \
                          .filter(ts.Sample.test == req.test) \
@@ -1900,6 +1903,7 @@ def v4_matrix():
     for req in plot_parameters:
         q = session.query(req.field.column, ts.Order.llvm_project_revision,
                           ts.Order.id) \
+            .select_from(ts.Sample) \
             .join(ts.Run) \
             .join(ts.Order) \
             .filter(ts.Run.machine_id == req.machine.id) \
@@ -1936,6 +1940,7 @@ def v4_matrix():
         q_baseline = session.query(req.field.column,
                                    ts.Order.llvm_project_revision,
                                    ts.Order.id) \
+                       .select_from(ts.Sample) \
                        .join(ts.Run) \
                        .join(ts.Order) \
                        .filter(ts.Run.machine_id == req.machine.id) \