slash command to update ignored rooms list

[Gnuxie]

Based on https://github.com/matrix-org/matrix-js-sdk/pull/2483 Doesn't actually ignore anything, just updates the list in the above PR

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[Yoric]

@Gnuxie Heads up: the type of getIgnoredInvites/setIgnoredInvites is changing.

[andybalaam]

@Gnuxie is this abandoned? Will close if so.