Typescript strict null checks pt1

[Palid]

Removed some unused variables (mostly private ones) and changed the types accordingly to strictNullCheck in tsconfig.

Please don't mind failing eslint so far, we need to figure this out slightly better. Maybe change APIs in case of the SettingsStore too (make new API and deprecate current one?)

Related to https://github.com/vector-im/element-web/issues/21967

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This change is marked as an internal change (Task), so will not be included in the changelog.

[Palid]

It seems that @SimonBrandner wants to take over this one, so please do! 😄

[SimonBrandner]

I don't have the time to handle this atm

[t3chguy]

Thanks for this but it has rotted beyond usability, some of these sites have already been fixed, rest will be addressed as part of https://github.com/vector-im/element-web/issues/21967