Don't logout user at login page

[bjoernricks]

Redirect to main page if the user is already logged in when visiting the
login page instead of clearing his token.

Checklist:

• Tests
• CHANGELOG Entry

[codecov]

Codecov Report

Merging #1508 into gsa-8.0 will increase coverage by <.01%.
The diff coverage is 75%.

@@             Coverage Diff             @@
##           gsa-8.0    #1508      +/-   ##
===========================================
+ Coverage    37.42%   37.42%   +<.01%     
===========================================
  Files          964      964              
  Lines        21828    21830       +2     
  Branches      6102     6083      -19     
===========================================
+ Hits          8169     8170       +1     
- Misses       12359    12360       +1     
  Partials      1300     1300

Impacted Files	Coverage Δ
gsa/src/web/pages/login/loginpage.js	89.85% <75%> (-1.19%)	⬇️

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