feature: sitemap generator job
we should use `sitemap` package to create a default job in `template/jobs` that creates/generates a sitemap every time the app boots.  finally the file should be generated to `build/sitemap.xml`.

this sitemap generator should be smart in the fact that it actually crawls our own website.  perhaps we should use another package for this.  it'd also solve the problem that pug does not cache templates until they are loaded for the first time - so as soon as the app would boot, agenda would trigger this sitemap job to run, it would cache and crawl all public pug pages (that'd google would normally see).  I'm not sure of a package to do the crawling except making some recursive function with `cheerio` to find all `$('a[href]')` on every page recursively and then load it and add to the sitemap array to be rendered to `sitemap.xml`

and we should also document for users in `README.md` how to submit their sitemap to google and verify their website

