commands: one grammar for !, merged commands, a settings sheet, two-level help

A `!` now means force or everything and nothing else (`:w!`, `:search!`,
`:export!`, `:logout!`). A command that did a second thing behind its bang
takes a word for it instead: `:space new|name|rm`, `:list name|rm`,
`:task [add]|edit`, `:upload seal`, `:import zip [/]`. `:card` while reading
a document makes a card about it, so `:card!` is gone.

Merged: `:export` downloads what has focus, a file as it is or a document as
markdown (`:download` folded in); `:share [off|new]` replaces `:unshare` and
`:link`, and the list's `s` and `y` give way to it and the info sheet;
`:noh` replaces `:clear`; `:vault [unlock|lock|passwd]` replaces `:unlock`,
`:lock` and `:passwd`; Esc in the list puts down what `m` picked up before it
closes the document, replacing `:unmark`.

The old spellings run as their new lines for one release through a table in
commands.ts; help and Tab leave them out, and the wiki lists them.

A settings sheet (`,` or `:settings`) holds the options, the vault, the
backup, the signed-in browsers, ssh keys and capture tokens. Each row runs
the command a line would. Bare `:set`, `:vault`, `:backup`, `:sessions`,
`:sshkey` and `:token` open it at their part.

`?` shows the keys and commands to start with, flagged in the keymap and the
command table; a second `?` (or `:help all`) shows all of them, and a filter
always searches all of them. Help rows are keyed by key and description, as
two contexts can share a label.

docs/wiki/Files-and-Search.md

@@ -17,7 +17,7 @@ Put files in:
 - drop them onto the list;
 - `:upload` picks files for the folder you are in, or for the document you
   are reading;
-- `:import` uploads a whole folder, and `:import!` a zip
+- `:import` uploads a whole folder, and `:import zip` a zip
   (see [Not losing things](Not-Losing-Things.md#import));
 - copy them in over the [SFTP mount](SFTP.md).
 
@@ -44,7 +44,7 @@ not stored. Nothing of a private key is sent back but its algorithm and size.
 
 ![A certificate read on the server](../images/file-certificate.png)
 
-Anything else is a download: `:download` (`:dl`) fetches the file under the
+Anything else is a download: `:export` fetches the file under the
 cursor, and `gx` opens it in a browser tab of its own. Files are renamed with
 `:name`, moved like documents, and deleted into the trash with `x`.