Fix graphiqlFetchUrl settings

[ParallelUniv3rse]

• Added ability to set-up graphiqlFetchUrl on settings page
• Fixed default graphiqlFetchUrl to point to CMS' base url instead of page base. That was causing issues when using Craft as headless and pointing Page base to the frontend site.

[ParallelUniv3rse]

Reverted the textField in settings, because I could not figure out how to add it properly without breaking the settings page. Would be nice to add an autosuggestField where users could choose aliases and environment variables as well.