Add validation to EventApplication model

[amakarudze]

• Add functionality to check if the organizer has an open application before submitting the application.
• Add functionality to check if the organizer has had an event in the past 6 months before submitting the application.
• Add tests to increase organize app coverage

[codecov-commenter]

Codecov Report

Merging #576 (0aec40a) into master (5620904) will increase coverage by 0.31%.
The diff coverage is 74.11%.

@@            Coverage Diff             @@
##           master     #576      +/-   ##
==========================================
+ Coverage   70.69%   71.00%   +0.31%     
==========================================
  Files          85       85              
  Lines        2692     2763      +71     
  Branches      267      274       +7     
==========================================
+ Hits         1903     1962      +59     
- Misses        725      738      +13     
+ Partials       64       63       -1     

Impacted Files	Coverage Δ
core/urls.py	100.00% <ø> (ø)
djangogirls/urls.py	88.88% <ø> (ø)
organize/admin.py	64.00% <0.00%> (-1.31%)	⬇️
organize/urls.py	100.00% <ø> (ø)
organize/views.py	70.21% <52.63%> (+21.72%)	⬆️
organize/models.py	85.36% <60.86%> (-5.82%)	⬇️
organize/forms.py	79.51% <92.85%> (+18.80%)	⬆️
core/validators.py	100.00% <100.00%> (+50.00%)	⬆️
djangogirls/settings.py	85.91% <100.00%> (ø)
sponsor/urls.py	0.00% <0.00%> (-100.00%)	⬇️
... and 4 more

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[RachellCalhoun]

lgtm