If your homeserver is configured with an experiment `widget_build_url`, this
will take over the functionality of the call buttons and turn them into a
general widget installer.
Note that this ditches all previously set width values, however this is probably acceptable for now. Trying to remain backwards compatible gets tricky on top of already tricky code, and the impact of Element forgetting widths is not as severe as forgetting which widgets were/are pinned.
The security key naming/practice was misguided, so let's call it what it is (a settings key) and abstract away the complexity to a new store.
Fixes https://github.com/vector-im/element-web/issues/15820 while we're here.
