Prevent auto-fill of saved credentials into HTTP auth fields

[hartwork]

Proposed approach to handle #605.

Tried with Firefox 58, Chromium 63, Chrome 66.

[codecov[bot]]

Codecov Report

Merging #609 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #609   +/-   ##
=======================================
  Coverage   80.99%   80.99%           
=======================================
  Files          45       45           
  Lines        2920     2920           
  Branches      177      177           
=======================================
  Hits         2365     2365           
  Misses        497      497           
  Partials       58       58

Impacted Files	Coverage Δ
cabot/cabotapp/views.py	71.3% <ø> (ø)	:arrow_up:

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[hartwork]

Cool!