Update scripts and docs to use yarn where appropriate

Most `npm` operations are replaced with `yarn`, which generally has better
behavior. However, steps like publish that write to the NPM registry are left to
`npm`, which currently handles these tasks best.

.gitignore

@@ -3,7 +3,7 @@ npm-debug.log
 /node_modules
 /lib
 
-# version file and tarball created by 'npm pack'
+# version file and tarball created by `npm pack` / `yarn pack`
 /git-revision.txt
 /matrix-react-sdk-*.tgz