UI: Fix scrollbar always present on page body

[silverwind]

28fcbf3 had reintroduced a small issue where the main page scrollbar was present even if there was no real scrollable content. To fix, I changed the height of the footer to flex-basis which works better than height, which fixes the issue in all Browsers except IE 11 where I had to additionally set the overflow on body to auto to display scrollbars only when needed.

[codecov-io]

Codecov Report

Merging #6177 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #6177   +/-   ##
=======================================
  Coverage   38.87%   38.87%           
=======================================
  Files         354      354           
  Lines       50184    50184           
=======================================
  Hits        19508    19508           
  Misses      27851    27851           
  Partials     2825     2825

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