Scuba plugin

Summary:
Scuba doesn't work when running on laptops, so developers often run a canary just to see that their logging works. This attempts to give a Scuba-like view (Scuba UI does a lot, I'm just showing a simple samples-like view) of the data you report in real time.

## Unity
* `ScubaFlipperPlugin` - very similar to our Pigeon/Logview plugins.
* `ScubaLogger` - notice that I'm ignoring the sample rate (weight) to give as much signal as possible since a local Unity usually doesn't have a lot of traffic/logs.

## Flipper
* `index.tsx`
  * Aggregates incoming data from Unity:
    * `messages` - all messages, keyed by the scribe category.
    * `columns` - keeps track of all the columns per table. This can grow over time (different messages can have different columns; Scuba does the same - using the union of all columns from all rows). I'm doing this here (and not while rendering a specific table) since this allows me to run the aggregation only once when a message arrives instead of every render (which React can do many of).
  * Chooses between one of the two following components.
* `ScubaTables.tsx` - shows all known tables so far, with a count of rows (just so I'll have something to show in table...). Each row has a button to view this table.
* `ScubaTable.tsx` - probably the only interesting part is not using explicit widths in the table (since I don't know what data I'll have). Instead, I just use `horizontallyScrollable` on the table and let it decide. It will squeeze everything in the window's width, but let's you expand/scroll as needed.

Differential Revision: D21715926

fbshipit-source-id: 6f1c4f7f77c3b047dcb35a4e9b8173a7378af198

desktop/static/icons.json

@@ -422,5 +422,8 @@
   ],
   "plus": [
     16
+  ],
+  "scuba": [
+    12
   ]
 }