#511 - Build 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.
- Rework `slPopoverHelp` to use the local copy of Confluence docs.
- Re-enable help for "Build universal" and "Build git".

A part of #511

/to @anthonyettinger or @seanbrookes please review
/cc @crandmck