Adds maplibre as a dependency, and behind a labs flag, lets users send and receive [MSC3488](https://github.com/matrix-org/matrix-doc/blob/matthew/location/proposals/3488-location.md) style location shares - with backwards compatibility with old school `m.location` `msgtype` location shares too.
For this to work, you have to define a valid maptile server and API in your config.json's `map_style_url`.
* Display started polls in timeline (without votes)
* Update i18n info
* Keep original background colour of poll options, even on hover
* Show full avatar above a poll message
