Fixes #7093.

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Since they are a language feature, they make the source code more easily
comprehensible than `if (typeof XXX === 'undefined') { XXX = ...; }`.

- Increase z-index so the subtitles display over tiles.
- Add a background to the subtitle text.
- In general make the subtitles narrower.

* Show subtitles when Jigasi sends transcription results in JSON

* fix: Import PropTypes from prop-types.

* apply feedback on initial PR

* Changed Object to Map, alphabetic ordering fixes ,css changes in transcription subtitles

* Sends Map of transcriptMessages as prop to Component

* Documentation fixes and uses config in redux state

* Minor doc fix

* rename feature 'transcription' to 'subtitles'

* Moves subtitles config to interfaceConfig and minor fixes

* minor lint fix