Better PWAs

Improved installable web apps for selected sites.

Open source extension that replaces or injects manifests for selected sites. This allows installation of some sites, and enhanced experience for others. Currently supported sites are: * the Slack app (app.slack.com), which has a manifest added and start URL set so the CCT (toolbar at the top showing the origin) doesn't show on Threads and other pages. A maskable icon is added to improve ChromeOS installation. * github (github.com) which is made into a Tabbed mode app with a maskable icon on ChromeOS * Sydney Morning Herald (www.smh.com) has a minimal UI manifest added. The default for sites is for the extension to always update the manifest. This is the case for Slack and the Sydney Morning Herald. Just go to these sites and install like you can a well configured PWA. github use CSP to prevent modification of their manifest. To enable the extension to provide an updated manifest, the extension needs to be enabled to block the CSP headers. Click the extension icon on github.com to enable this. Once the extension is activated for the origin, reload the page and install the app. Then disable the extension to restore normal CSP. Please use the project github pages* to suggest sites to support. PRs welcome. * https://github.com/benfredwells/betterPWAs/issues