Add loading state to compare branches and continue button

This is a rebased MR from a community contributor's MR: !84649

It was needing a rebase and gone stale for a few weeks. Helping it move forward this way.

Original author: @stijn98s

What does this MR do and why?

Disables and added spinner to Compare branches and continue button to show platfom is busy.

Screenshots or screen recordings

Screencast_from_07-04-22_22_33_46

MR acceptance checklist

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