Few minor auth flow security and performance improvements

[randomir]

Updating the auth flow according to OAuth 2.0 Security Best Current Practice, motivated by #605.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 88.48%. Comparing base (535a9e7) to head (1245234).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #607 +/- ## ======================================= Coverage 88.47% 88.48% ======================================= Files 33 33 Lines 4356 4359 +3 ======================================= + Hits 3854 3857 +3 Misses 502 502 ```

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