improve interface to more card metadata+issues

upgrade Footer's "Cards: ..." to a single interface to all kinds of related cards, e.g.:

• plus elements (currently shown)
• cards tagged with any of the same tags, "tag1: foo bar<newline>tag2:baz dap"
• cards of which this is an element
• incoming (backlinks), outgoing

combine with Related tab?

This is still fuzzy.