Small fixes to new layout

[Redande]

• Better contrast for language selection in course editor
• Fix study module links in frontpage
• Rotate arrows in outbound links to point northeast instead of left
• Fix creative commons link in teachers manual

[codecov]

Codecov Report

Merging #1250 (07bde9e) into master (7f68e1d) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1250   +/-   ##
=======================================
  Coverage   66.43%   66.43%           
=======================================
  Files         144      144           
  Lines        6042     6042           
  Branches     1434     1434           
=======================================
  Hits         4014     4014           
  Misses       1889     1889           
  Partials      139      139           

Flag	Coverage Δ
backend	66.43% <ø> (ø)

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