Add a placeholder string when no resources are available/no channels on devices

[marcellamaki]

Summary

This adds a common string that can be used across Kolibri for situations where there are no channels imported on the device and therefore no resources that are available for the user - whether this is for viewing, or for selecting (i.e. lesson and quiz creation).

References

Fixes #13021

Reviewer guidance

1. Remove all channels from the device, and ensure that the string displays on the /lessonstemp path when opening the "manage resources" side panel.
2. When their are channels, the channel cards should display correctly.

[github-actions]

Build Artifacts

Asset type	Download link
PEX file	kolibri-0.18.0a0.dev0_git.397.gbe09a87f.pex
Windows Installer (EXE)	kolibri-0.18.0a0.dev0+git.397.gbe09a87f-windows-setup-unsigned.exe
Debian Package	kolibri_0.18.0a0.dev0+git.397.gbe09a87f-0ubuntu1_all.deb
Mac Installer (DMG)	kolibri-0.18.0a0.dev0+git.397.gbe09a87f.dmg
Android Package (APK)	kolibri-0.18.0a0.dev0+git.397.gbe09a87f-0.1.4-debug.apk
Raspberry Pi Image	kolibri-pi-image-0.18.0a0.dev0+git.397.gbe09a87f.zip
TAR file	kolibri-0.18.0a0.dev0+git.397.gbe09a87f.tar.gz
WHL file	kolibri-0.18.0a0.dev0+git.397.gbe09a87f-py2.py3-none-any.whl

[radinamatic]

/lessonstemp path working as expected with the new string:

[radinamatic]

LGTM! 👍🏽 💯

[AlexVelezLl]

Code changes makes sense to me!