IE11 compatibility fixes

[rtibbles]

Summary

• Removes use of Promise.finally from Kolibri application code as polyfilling it within babel/preset-env has led to difficult to resolve issues
• Consolidates and makes explicit our browserslist - and removes use of 'last 2 versions' so that we don't experience progressive browser support drift
• Restores explicit polyfills for Hashi, as using babel/preset-env entry bloats the code enormously and usage results in runtime errors

References

• Fixes Promise.finally is not polyfilled properly for IE11 #8153

Reviewer guidance

• Testing that the learn page loads on IE11
• Checking that HTML5 apps run on IE11

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Testing checklist

• Contributor has fully tested the PR manually
• If there are any front-end changes, before/after screenshots are included
• Critical user journeys are covered by Gherkin stories
• Critical and brittle code paths are covered by unit tests

PR process

• PR has the correct target branch and milestone
• PR has 'needs review' or 'work-in-progress' label
• If PR is ready for review, a reviewer has been added. (Don't use 'Assignees')
• If this is an important user-facing change, PR or related issue has a 'changelog' label
• If this includes an internal dependency change, a link to the diff is provided

Reviewer checklist

• Automated test coverage is satisfactory
• PR is fully functional
• PR has been tested for accessibility regressions
• External dependency files were updated if necessary (yarn and pip)
• Documentation is updated
• Contributor is in AUTHORS.md

[radinamatic]

IE11 is not blocked anymore by the Promise polyfill, goes smoothly through the setup wizard and channel import, Learn page looks OK.
There are issues loading resources, but it seems to be a general Windows backend issue (not extant on Linux with asset from the same build), we are going to address it as a separate issue/PR.

[radinamatic]

Polyfill issue fixed, let's address the broken resource loading on Windows separately.