Support routing in the web client

[pixelspark]

Allow specific pages to be selected using URL routing:

• /welcome
• /data/[db]/[q]
• /blocks/[blockid]
• /identity/[pubhash]

Use window.history.pushState.

[pixelspark]

Needs a global routing or state object from which current selections etc. are loaded. The object should also support updating, e.g.

{
mainTab: "identities",
selectMainTab(name) { this.$set('mainTab', name); updateURL(); }
updateURL() { window.location.hash = mainTab+"/"+... }
}

[pixelspark]

Implemented for tabs, can be extended to reach within tabs. At some point we should switch to using e.g. vue-router.