Fix 500 error on /contest/<code>/leave

[int-y1]

The 500 error: visit https://dmoj.ca/contest/cheerio1j/leave while logged in.

The issue is that a GET request goes to BaseDetailView.get then crashes at self.render_to_response. https://github.com/django/django/blob/5c3300027b30c1b498d99010f7d618316f685045/django/views/generic/detail.py#L103-L108

In the new version, GET requests receive error 405.

Also, ContestJoin doesn't need BaseDetailView because ContestJoin implements its own get method.

[codecov-commenter]

Codecov Report

Merging #1956 (4a222fe) into master (966b37d) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1956   +/-   ##
=======================================
  Coverage   46.46%   46.46%           
=======================================
  Files         236      236           
  Lines       13114    13114           
=======================================
  Hits         6093     6093           
  Misses       7021     7021           

Impacted Files	Coverage Δ
judge/views/contests.py	29.23% <100.00%> (ø)

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[Ninjaclasher]

LGTM

[Xyene]

LGTM, thanks!