Fix buttons and improve handling of certificates when Safari is not the default browser

[MatteoPologruto]

Please check if the PR fulfills these requirements

• The PR has no duplicates (please search among the Pull Requests
  before creating one)
• Tests for the changes have been added (for bug fixes / features)

• What kind of change does this PR introduce?

Bug fix

• What is the current behavior?

"Manage HTTPS certificate" buttons do not work correctly and some cases in which the user's default browser is not Safari are poorly handled.

• What is the new behavior?

The above mentioned behavior is fixed.

• Does this PR introduce a breaking change?

No

• Other information:

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 73 lines in your changes are missing coverage. Please review.

Project coverage is 20.66%. Comparing base (222a505) to head (c461c40).

Files	Patch %	Lines
main.go	0.00%	39 Missing ⚠️
systray/systray_real.go	0.00%	24 Missing ⚠️
certificates/install_default.go	0.00%	5 Missing ⚠️
utilities/utilities.go	0.00%	3 Missing ⚠️
certificates/certificates.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #949      +/-   ##
==========================================
- Coverage   20.71%   20.66%   -0.06%     
==========================================
  Files          42       42              
  Lines        2592     2599       +7     
==========================================
  Hits          537      537              
- Misses       1958     1965       +7     
  Partials       97       97              

Flag	Coverage Δ
unit	20.66% <0.00%> (-0.06%)	⬇️

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