Offline mode / PWA support (WORK IN PROGRESS)

[amcgee]

This is a placeholder for Progressive Web App and "Offline Mode" support specification.

This blocks a highly-requested feature for Dashboards (DHIS2-8341) as well as migration of Tracker Capture and Capture apps to the platform (as they currently support some forms of metadata caching or offline modes)

There are four main components to this effort:

• Enable a Service Worker and Web Application Manifest to "activate" PWA
• Pre-load the selected data (certain dashboards, for instance) and save it in a persistent store (localstorage / indexeddb / websql) for later recall (related to dhis2/app-platform#372)
• Implement UI changes for “offline mode” - disable links to online-only apps, support an online/offline toggle, disable online-only features
• Update application CSS and functional UI to support small-form-factor devices (phones and tablets)

All four of these components have implications for the platform, and the latter two will also have application-specific requirements.