Add record tab for displaying channels.

[demiankatz]

This PR creates a tab to embed channels on the record page. It is configured to be disabled by default; uncomment the relevant line(s) in RecordTabs.ini to try it out.

This also adds a missing translation: the text "Channels" is already used in breadcrumbs but was missing from language files.

The ChannelLoader code has also been simplified to take advantage of constructor property promotion.

TODO:

• Discuss: do we need a way to configure different channels in the tab vs. the normal "record channels" context?
• Add test coverage
• Discuss: should we deprecate the "similar items" tab in favor of the richer channels tab? We could create a default configuration of the Channels tab that behaves almost exactly like similar items in order to take its place. (This might make the most sense as a follow-up action after merging this PR).