controllers/user/session: Replace r2d2 with deadpool

[Turbo87]

This changes the order of operations a bit. Previously, we were first doing the GitHub requests and then connecting to the database. Now, we are first connecting to the database before doing the sync OAuth exchange with GitHub. This allows us to move from spawn_blocking() to conn.interact() though, which in turn allows us to get rid of the sync database connection pools soon.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 87.61%. Comparing base (48ba166) to head (ddf8c41).
Report is 1 commits behind head on main.

Files	Patch %	Lines
src/controllers/user/session.rs	0.00%	4 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #8440   +/-   ##
=======================================
  Coverage   87.61%   87.61%           
=======================================
  Files         272      272           
  Lines       26330    26329    -1     
=======================================
  Hits        23068    23068           
+ Misses       3262     3261    -1     

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[Turbo87]

@LawnGnome I'll go ahead and merge this to unblock the next PR, but feel free to review after-the-fact :)