Fix hardcoded references to xxxxx.3di.live.

[jpprins1]

Some clients have all their 3Di web interfaces (3Di Live, 3Di Management) on different URLs than the default ones.

The pattern is always the same:

• API: https://api.{base_url}
• 3Di Management: https://management.{base_url}
• 3Di Live: https://www.{base_url}

The default base URL is 3di.live

To do:

• [x] In the plugin settings, let users set the base URL instead of the API base URL
• [x] If the user-defined base URL starts with https://api., remove that part (so users do not have to do this manually after updating their plugin).
• [x] Change the following hardcoded URLS to use the user-defined base URL

threedi_models_and_simulations/settings.py:89:        webbrowser.open("https://management.3di.live/personal_api_keys/")
threedi_models_and_simulations/widgets/threedi_dockwidget.py:225:        webbrowser.open("https://management.3di.live/")

• [x] Check if the following hardcoded URLs need to be changed to use the user-defined base URL

  threedi_models_and_simulations/settings.py:19:    PRODUCTION_API_URL = "https://api.3di.live"
  threedi_models_and_simulations/settings.py:20:    STAGING_API_URL = "https://api.staging.3di.live"

[leendertvanwolfswinkel]

@ldebek please merge