Fix CopyClassModal handle removed coaches

[nucleogenesis]

Summary

Fixing a bug described by @pcenov during his testing on #13865

In the ClassEditPage, when a user would remove a coach, the state for the currentClass was not being updated. Manually updating it here fixes the reported issue.

This only affected the coaches because, despite there being classLearners and classCoaches properties on the module's state, the ClassCopyModal fetches the learners for the target class, but uses the currentClass.coaches value passed into it's classToCopy prop.

This isn't really the ideal solution, but it'll avoid introducing a bug to our new-to-this-page class copy functionality.

Reviewer guidance

Go to a class's details page and, in general:

• Copy the class by clicking the "Options" dropdown - be sure that the same number of learners & coaches (and the same actual users) are in the new copy
• Open the "Copy class" modal, cancel it, then remove coaches and/or learners, go to copy again and verify the numbers shown reflect the users who remain in your class. Copy them and verify that the users you'd removed aren't included in the copied class.

[rtibbles]

This should work, if I am understanding the code correctly!

[github-actions]

Build Artifacts

Asset type	Download link
PEX file	kolibri-0.19.0b1.dev0_git.61.g303784f3.pex
Windows Installer (EXE)	kolibri-0.19.0b1.dev0+git.61.g303784f3-windows-setup-unsigned.exe
Debian Package	kolibri_0.19.0b1.dev0+git.61.g303784f3-0ubuntu1_all.deb
Mac Installer (DMG)	kolibri-0.19.0b1.dev0+git.61.g303784f3.dmg
Android Package (APK)	kolibri-0.19.0b1.dev0+git.61.g303784f3-0.1.6-debug.apk
Raspberry Pi Image	kolibri-pi-image-0.19.0b1.dev0+git.61.g303784f3.zip
TAR file	kolibri-0.19.0b1.dev0+git.61.g303784f3.tar.gz
WHL file	kolibri-0.19.0b1.dev0+git.61.g303784f3-py2.py3-none-any.whl

[pcenov]

Thanks @nucleogenesis - the coach count is updated correctly now.