add a basic Javascript UI

The Javascript is pretty amateurish I'm sure but at least it's something to
iterate from. It's already much more pleasant for browsing through videos in
several ways:

* more responsive to load only a day at a time rather than 90+ days
* much easier to see the same time segment on several cameras
* more pleasant to have the videos load as a popup rather than a link
  that blows away your position in an enormous list
* exposes the fancier .mp4 generation options: splitting at lengths
  other than the default, trimming to an arbitrary start and end time,
  including a subtitle track with timestamps.

There's a slight regression in functionality: I didn't match the former
top-level page which showed how much camera used of its disk allocation and
the total duration of video. This is exposed in the JSON API, so it shouldn't
be too hard to add back.

guide/install.md

@@ -17,10 +17,7 @@ from source.
 
 Moonfire NVR is written in the [Rust Programming
 Language](https://www.rust-lang.org/en-US/). In the long term, I expect this
-will result in a more secure, full-featured, easy-to-install software. In the
-short term, there will be growing pains. Rust is a new programming language.
-Moonfire NVR's primary author is new to Rust. And Moonfire NVR is a young
-project.
+will result in a more secure, full-featured, easy-to-install software.
 
 You will need the following C libraries installed:
 
@@ -55,6 +52,8 @@ all non-Rust dependencies:
 Next, you need Rust 1.17+ and Cargo. The easiest way to install them is by following
 the instructions at [rustup.rs](https://www.rustup.rs/).
 
+Finally, building the UI requires [yarn](https://yarnpkg.com/en/).
+
 You can continue to follow the build/install instructions below for a manual
 build and install, or alternatively you can run the prep script called `prep.sh`.
 
@@ -85,9 +84,12 @@ For instructions, you can skip to "[Camera configuration and hard disk mounting]
 
 Once prerequisites are installed, Moonfire NVR can be built as follows:
 
+    $ yarn build
     $ cargo test
     $ cargo build --release
     $ sudo install -m 755 target/release/moonfire-nvr /usr/local/bin
+    $ sudo mkdir /usr/local/lib/moonfire-nvr
+    $ sudo cp -R ui-dist /usr/local/lib/moonfire-nvr/ui
 
 ## Further configuration