Move download string to core strings

[MisRob]

Summary

Moves the existing "Download" string to core strings as we will soon need it in the content renderer (see #9856)

Reviewer guidance

• Is the placement of the string fine?
• Bonus points: I did this, but with the string freeze coming fast, it'd be much appreciated if you checked related references in Remote Content Browsing: Update Content Renderer for downloading and streaming #9856 and checked that this is indeed the only string we need for this issue

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

[github-actions]

Build Artifacts

Asset type	Download link
PEX file	kolibri-0.16.0a7.dev0_git.130.g926caf7e.pex
Windows Installer (EXE)	kolibri-0.16.0a7.dev0+git.130.g926caf7e-unsigned.exe
Debian Package	kolibri_0.16.0a7.dev0+git.130.g926caf7e-0ubuntu1_all.deb
Mac Installer (DMG)	kolibri-0.16.0a7.dev0+git.130.g926caf7e-0.3.0.dmg
Source Tarball	kolibri-0.16.0a7.dev0+git.130.g926caf7e.tar.gz
WHL file	kolibri-0.16.0a7.dev0+git.130.g926caf7e-py2.py3-none-any.whl

[rtibbles]

This does indeed seem to be the only string we need only for this specific place.

I do note that we need a 'Disconnected' string here and elsewhere, but let's merge this and make sure we have that too.

[MisRob]

I do note that we need a 'Disconnected' string here and elsewhere, but let's merge this and make sure we have that too.

@rtibbles this will be added by @jredrejo in the scope of #9859 (related Slack thread)

cc @marcellamaki @radinamatic