fix: restore using local path web logo

[juanpicado]

After release 5.x the configuration web.config.logo loading a local file is unusable, this PR restore the 4.x behavior, loading a logo from local path and https url.

Additionally add some checks, if the file exist and is readable, otherwise fallback to the original logo and will print a warning in the log in such case.

web:
  title: Verdaccio
  # logo: /home/user/Downloads/vue_logo.png
  # or https (http is not longer valid)
  # logo: https://somedomain/test.png