Update login page background

[maciaszczykm]

What this PR does / why we need it:

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #2817

Special notes for your reviewer:

I'll replace Dex image too in the core repo.

Release note:

Updated login page background.

[kubermatic-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: maciaszczykm

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• OWNERS [maciaszczykm]

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[maciaszczykm]

/assign @floreks

[codecov]

Codecov Report

Merging #2849 (0863792) into master (f9494c0) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2849   +/-   ##
=======================================
  Coverage   52.35%   52.35%           
=======================================
  Files         234      234           
  Lines        8188     8188           
  Branches      939      939           
=======================================
  Hits         4287     4287           
  Misses       3898     3898           
  Partials        3        3           

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[floreks]

/lgtm

[kubermatic-bot]

LGTM label has been added.

Git tree hash: f3986d6910f6c10e4973a8e65f939a6de2bc765f