[22.05]Fix absolute path generation util

[dannon]

Just shifts slashcleanup to only the composed portion to avoid deduplicating '/' in protocol.

Issue reported by @nekrut on matrix; to test this use the dataset 'copy' function and note that before you'd get a url like:

https:/usegalaxy.org/api/datasets/f9cad7b01a4721358cfdfdaa2fac173c/display?to_ext=tabular

(note the single slash in https:/)

How to test the changes?

(Select all options that apply)

• I've included appropriate automated tests.
• This is a refactoring of components with existing test coverage.
• Instructions for manual testing are as follows:
  1. Try copying a dataset url with the link icon

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[github-actions]

This PR was merged without a "kind/" label, please correct.