Factor `rsync` invocation into a function.

(This is not terribly useful yet, but will be once more rsync
invocations start happening.)

Add KaTeX (the version currently used by the server) as a dependency.

Pre-sync (and, if they ever exist, post-sync) scripts will be located
in a subdirectory of tools/, named `build-webview.d` (following the
modern Linux convention of placing configuration file-fragments or
auxiliary scripts in similarly named directories).

(No such scripts are included in this particular commit.)

Add a pre-build script to copy the KaTeX fonts and support files into
the staging directory for later bundling/packaging.

Add links to those files in the WebView HTML head.

* N.B.: Although the installation directions just say to include
  katex.[min.]js unconditionally, the KaTeX JavaScript isn't actually
  needed just to display existing KaTeX blocks -- only to render new
  ones.

(At this point, most inline equations and many block equations are
visible with their intended formatting.)

Wrap each block KaTeX equation in a small shim allowing it to be
scrolled horizontally.

Since the shim doesn't come with a visible scrollbar, add a visible
border in lieu thereof.

... and eliminate the legacy meaning of an unspecified second parameter.

This potentially involves some minor functional changes to
`handleInitialLoad`, in that the order of actions therewithin is
changed -- but if those actions don't commute, then we should probably
have been rewriting the source HTML before scrolling anyway.

KaTeX `.frac-line`s don't show up on some of our Android test devices
and emulators. (This seems to be a perennial problem for KaTeX. See
the commit-internal comments for references.)

Resolve this by forcing all such lines to be 1px wide, disregarding
the TeXbook spec.

Mixing permanent Git-controlled files and temporary build-"generated"
files in the same directory is... probably not a great idea.

Instead, use a dedicated external directory as a staging directory,
and treat `src/webview/static/` as just another source for files.

Using the `--delete-excluded` option everywhere causes the auxiliary
scripts to be noncommutative: in particular, the `00-base` script will
delete the preexisting `katex` subdirectory from the staging
directory, even when there was no need.

Remove it from the common invocation, and specify it explicitly only
for the separate staging-to-destination rsync.

----

N.B.: This leaves open the possibility that stale files will remain in
the staging directory, forgotten yet quietly relied upon. This will
need to be addressed -- but addressing it isn't expected to involve
significant changes to the auxiliary build scripts.