Rename intro.js and outro.js to have .txt extensions

This will make it easier to e.g. pass all of src/ to jsduck
without it tripping on parse errors.

This also fixes the static jsduck test in Jenkins since we can't
use /dist/ without building a distribution from source first,
and we can't easily pass all files except intro/outro to jsduck
because it doesn't support wildcards and doesn't support excluding
files (we'd have to list each directory and file separately).

Change-Id: I7481ddb210c963063a9483ddc8b3ac1fc0de056e