Implement web install using fastboot.js

This implements the WebUSB-based web installer using fastboot.js to
act as a fastboot client. A bare minimum UI with a plain-text
status/progress caption for each step is included, as well as a plain
button to trigger it and basic error handling.

WebADB has been removed now that we are only using fastboot.js.

Initial features:

- Unlocking and locking the bootloader
- Downloading the latest GrapheneOS release available for the device
- Flashing the factory images zip
- Reusing USB connections

static/grapheneos.css

@@ -150,6 +150,11 @@ footer a, footer a:visited {
     list-style-type: none;
 }
 
+.error-text {
+    /* Baseline Material error color */
+    color: #b00020;
+}
+
 /* latin */
 @font-face {
     font-family: 'Roboto';