Add /jumptodate slash command

[MadLittleMods]

Add /jumptodate 2021-08-22 slash command.

/jumptodate <date> will parse whatever Date.parse can handle:

Date.parse('2018-07-22')
Date.parse('2018-07') //July 1st 2018, 00:00:00
Date.parse('2018') //Jan 1st 2018, 00:00:00
Date.parse('07/22/2018')
Date.parse('2018/07/22')
Date.parse('2018/7/22')
Date.parse('July 22, 2018')
Date.parse('July 22, 2018 07:22:13')
Date.parse('2018-07-22 07:22:13')
Date.parse('2018-07-22T07:22:13')

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Relies on this matrix-js-sdk PR to add MSC3030 /timestamp_to_event support: matrix-org/matrix-js-sdk#2071

Jump to date headers are being worked on in #7339

Fix element-hq/element-web#7677

Part of MSC3030: matrix-org/matrix-spec-proposals#3030

Experimental Synapse implementation added in matrix-org/synapse#9445

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Here's what your changelog entry will look like:

✨ Features

• Add /jumptodate slash command (#7372). Fixes URL for an arbitrary day of history and navigation for next and previous days element-hq/element-web#7677. Contributed by @MadLittleMods.

Preview: https://61b9de3d1ac3eedfdcf91598--matrix-react-sdk.netlify.app
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[t3chguy]

[MadLittleMods]

Thanks for the review @t3chguy 🐪

Great shout on improving the error message 👌