v3.13.0 — A-Cell rebuilt around sessions, tapes and the phone (PR #94)

Also carries the v3.12.0 work, which was never released on its own.

A-Cell

• Three modes: Prep (Session prep, Players, Evidence, Music, Notes, Sheet, Cells, ID creator), Play (a desk you arrange) and Debrief.
• A session keeps its notes for Play: vault notes, Google Docs (read-only in Play) and uploaded HTML pages, in order, each removable.
• Notes vault synced from the phone, with [[ link suggestions, Undo/Redo, Back/Forward, bold and red links.
• Tapes: a cassette per operation or Cell with tracks, links, loops and pads.
• One compact player on the Play desk and the phone's Pads: channels, the tape's tracks on the green screen, Music / Loops / Effects levels, On air now, and "Play on this device".
• Sheet as Table or Cards with Filter, Columns and Select. Cells take up to three channels.
• A-Cell on a phone: its own layout with tabs, + post-its, a dashboard, Pads, Initiative and the notebook.

Fixes

• A tape no longer flips between two tracks.
• The progress bar works on a phone.
• The player stays on Pads.
• Pads only loads this session's tape.
• New NPCs show in Initiative at once.
• Stylesheets are no longer served stale after an update.