Restore iOS support

[kollivier]

...as iOS blockers are either resolved or no longer reproduce.

Summary

This change restores Mobile Safari as a supported platform, as #1049 was addressed and the other two blockers don't reproduce (#1046 and #2158).

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Contributor Checklist

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

Testing:

• Contributor has fully tested the PR manually
• If there are any front-end changes, before/after screenshots are included
• Gherkin stories have been updated
• Unit tests have been updated

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.rst

[radinamatic]

Dang, I have no iOS device to test this, will have to add it to my shopping list soon... 😕

[kollivier]

If you have a Mac and XCode, you can start Kolibri on your Mac and then use the iOS Simulator to start a virtual iOS device and run Safari. That is how I was testing because I didn't have any devices running iOS 9, which is where the bug was triggering.

[radinamatic]

Ah, good suggestion @kollivier, didn't think of that route...
In that case I'll have to undust my MacMini, it's been dormant for months 😛

[codecov]

Codecov Report

Merging #4671 into develop will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop    #4671   +/-   ##
========================================
  Coverage    51.42%   51.42%           
========================================
  Files          833      833           
  Lines        25088    25088           
  Branches      3303     3303           
========================================
  Hits         12901    12901           
  Misses       11498    11498           
  Partials       689      689

Impacted Files	Coverage Δ
...ore/context_processors/custom_context_processor.py	96.55% <ø> (ø)	⬆️

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[rtibbles]

Code looks good to me, so I'll leave it to testing to discern this PR's worth!

[indirectlylit]

thanks - merging in so we can start testing

@kollivier would you mind updating the browserslist file re: #4672