Use babel-plugin-dedent in Review Board.

Review Request #151 — Created Nov. 15, 2016 and submitted — Latest diff uploaded

Review Board
In another change, I've added an npm dependency to Djblets to install
babel-plugin-dedent, which allows us to use ES6 template strings without
filling our compiled output with a ton of ugly spaces. This change enables it
for Review Board as well.

Testing done:
- Ran ` develop` and saw babel-plugin-dedent get installed correctly.
- Used the dedent tag in extension code.

Reviewed at