Update markdown_controller.js

[d-buchmann]

Don't handle links as external by default. Instead distiguish internal (relative) and external (absolute) links.
That way, if you link to filters in your Part-DB banner, you won't end up with lots of tabs open at the end of the day.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 59.71%. Comparing base (b8d5b83) to head (e5a7ddb).

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #939      +/-   ##
============================================
- Coverage     59.73%   59.71%   -0.02%     
  Complexity     6036     6036              
============================================
  Files           529      529              
  Lines         20588    20588              
============================================
- Hits          12298    12295       -3     
- Misses         8290     8293       +3     

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