Add RSSI value to display status bars

Created from #522

Currently the WiFi symbol varies the number of bars populated based on predefined RSSI levels. It's mostly only useful for "Great reception" [2bars+dot] or "not great" [1bar+dot] or "questionable" [0bars+dot] (where mine live most of the time).
Similar to🛜 with 1less top bar.

It would be preferred to include the actual RSSI number alongside the symbol.

Similarly the battery level could be covered. Conversation:
> **[Tyeth]**
I now question if we need a statusbar proto, and then can turn on/off the numeric values for bat+rssi, plus the statusbar as a whole. Hmm, now I'm thinking the title (io username) could be customised too (defaults to {{aio_user}} ) (edited) 
**Brent Rubell  [7:29 PM]**
Too complex
[7:29 PM]We could control it with a separate feed, though
[7:30 PM]Esp if WiFi and Batt values are feeds.
**Tyeth  [7:30 PM]**
Just thinking the battery icon has same issue as rssi (no useful value). Both want to be feeds optionally (edited) 
**Brent Rubell  [7:30 PM]**
keep in mind this idea was prior to actions/templating
[7:30 PM]Status bar could be a string with liquid template hooked up to component feeds (edited) 
[7:31 PM]or it could be the default txt
**Tyeth  [7:31 PM]**
So you'd have battery icon placeholder etc in the template value on the feed. That would potentially be updated by actions
**Brent Rubell  [7:32 PM]**
Yeah

No source file, test, or implementation entry point is named in the issue. First trace how the WiFi status symbol and RSSI levels are rendered, then determine whether the requested numeric RSSI belongs in the existing status bar or requires the unresolved feed/template design; done means the chosen display behavior is specified and covered by tests.

Written by the indexing model from the issue text.