Change default presidential year to 2020

[lbeaufort]

Summary (required)

• Resolves Change default presidential candidate data table to 2020 #2152
  Change default presidential year to 2020.

Impacted areas of the application

List general components of the application that this PR will affect:

• Presidential candidate data table http://localhost:8000/data/candidates/president/
• DEFAULT_PRESIDENTIAL_YEAR constant

Screenshots

Before

After

Related PRs

Fixing 2020 and future election totals: fecgov/openFEC#3219

[codecov-io]

Codecov Report

Merging #2156 into develop will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop    #2156   +/-   ##
========================================
  Coverage    79.17%   79.17%           
========================================
  Files           45       45           
  Lines         3280     3280           
  Branches       490      490           
========================================
  Hits          2597     2597           
  Misses         683      683

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