Make frontend work with non-root URL

[schmengler]

When using a subdirectory as PUBLIC_URL, e.g. https://example.com/yopass, some URLs were not working:

• translations and the favicon were not loaded because they used / as base URL
• result links were assuming scheme://host/ as base URL
• links in header used / as base URL

With this PR, the environment variable PUBLIC_URL is respected in all these places if set. If not, fall back on previous behavior

[codecov]

Codecov Report

Merging #732 (1204d6c) into master (10f9113) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #732   +/-   ##
=======================================
  Coverage   78.81%   78.81%           
=======================================
  Files           6        6           
  Lines         354      354           
=======================================
  Hits          279      279           
  Misses         49       49           
  Partials       26       26           

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[jhaals]

Thanks a lot for adding support for this 👍