convert layout to cards in prep for a sidebar

/depends on !15 (merged) and !16 (closed)

Patch in !16 (closed) removes original BS stylesheet, instead it provides dual colour theme based on BS5. This introduces the following risks:

• maintainer bases his work on BS 5, but we don't know what will happen when we upgrade to next version of BS
• if necessary we would be patching a "patch" rather than upstream, just additional potential problem

We could instead:

• borrow JS code to use prefs for theme switching/detecting
• stay with upstream bootstrap for light theme with our individual changes if needed
• build dark theme on top of upstream BS that we control and update when we want