Add WCIPEG account merging

[fluix-dev]

trafficstars

Merge Process:

• WCIPEG user logs in to DMOJ with their account and sees a button on their profile to merge.
• WCIPEG user fills in the name of a native DMOJ user to merge into.
• An email is sent to the DMOJ user to accept the merge.
• WCIPEG user clicks this link, has their data merged, get logged out, and deactivated.
• DMOJ user now has the WCIPEG user's data and can be logged in to as before.

TODO:

• [x] Request Form
  • [x] Scary JavaScript confirm dialog
  • [ ] ~~Separate AJAX endpoint to filter out peg users with select2 (or js that does this frontend)?~~
  • [x] Generating token
  • [x] Email sending functionality
  • [x] ReCaptcha
• [x] Activation form
  • [x] Verify acc_peg login
  • [x] Log out user
  • [x] Merge user
  • [x] generic_message success message
• [x] PEGUser model
  • [x] user OneToOneField to WCIPEG account
  • [x] merge_user OneToOneField (nullable) to DMOJ account

[codecov-commenter]

Codecov Report

Merging #1448 into master will increase coverage by 0.21%. The diff coverage is 46.23%.

@@            Coverage Diff             @@
##           master    #1448      +/-   ##
==========================================
+ Coverage   45.84%   46.05%   +0.21%     
==========================================
  Files         208      214       +6     
  Lines       11968    12152     +184     
==========================================
+ Hits         5487     5597     +110     
- Misses       6481     6555      +74     

Impacted Files	Coverage Δ
peg_merge/apps.py	0.00% <0.00%> (ø)
peg_merge/migrations/0001_initial.py	0.00% <0.00%> (ø)
peg_merge/models.py	0.00% <0.00%> (ø)
judge/models/profile.py	92.26% <33.33%> (-1.08%)	:arrow_down:
peg_merge/views.py	50.98% <50.98%> (ø)
peg_merge/forms.py	60.86% <60.86%> (ø)
dmoj/urls.py	90.00% <100.00%> (+0.52%)	:arrow_up:
judge/signals.py	68.00% <0.00%> (-8.20%)	:arrow_down:
judge/views/submission.py	32.91% <0.00%> (-0.25%)	:arrow_down:
... and 15 more

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[Xyene]

This is patched in prod and will remain so, but let's close it out to decrease open PR clutter.