Use isort to keep imports consistent

[maratori]

• add make isort
• validate imports in linter
• can't use flake8-isort because of flake8-isort 100 times slower than isort gforcada/flake8-isort#101

[codecov-commenter]

Codecov Report

Merging #105 (2d5644b) into master (1f0b6b1) will decrease coverage by 0.01%.
The diff coverage is 88.09%.

@@            Coverage Diff             @@
##           master     #105      +/-   ##
==========================================
- Coverage   37.44%   37.43%   -0.02%     
==========================================
  Files          67       67              
  Lines        4305     4304       -1     
==========================================
- Hits         1612     1611       -1     
  Misses       2693     2693              

Impacted Files	Coverage Δ
lona/command_line/handle_command_line.py	0.00% <0.00%> (ø)
lona/middlewares/lona_messages.py	0.00% <0.00%> (ø)
lona/middlewares/sessions.py	0.00% <0.00%> (ø)
lona/shell/commands/lona_routes.py	0.00% <0.00%> (ø)
lona/shell/commands/lona_views.py	0.00% <0.00%> (ø)
lona/__init__.py	80.00% <100.00%> (ø)
lona/app.py	29.41% <100.00%> (ø)
lona/client_pre_compiler.py	35.55% <100.00%> (ø)
lona/command_line/run_server.py	25.00% <100.00%> (ø)
lona/default_settings.py	100.00% <100.00%> (ø)
... and 16 more

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[fscherf]

@maratori Great change! Thanks for that :)