Gulpfile: download help assets

Implement a new build step 'build-help-assets' that downloads
help pages from Confluence and stores them in `client/www/help/*.json`.

The list of pages to download is configured in `client/help.json`.

The content of help pages is processed using `cheerio` and
all relative Confluence links are resolved to absolute ones.
client: use the local help files

Rework `slPopoverHelp` to use the local copy of Confluence docs.

Re-enable help for "Build universal" and "Build git".
Merge pull request #534 from strongloop/feature/build-help-assets

#511 - Build help assets