Add a URL panel to the right panel

[Arnei]

Adds a new panel the right menu that, akin to "Files", shows all messages in the room containing at least one URL. This would help users find previously posted URLs without having to come up with the correct search term(s) to find them.

Example images:

Code-wise this more of a proof of concept. UrlPanel.tsx is almost completely copy and pasted from FilePanel.tsx. UrlPanel.tsx should probably either be stripped down or both of them should be abstracted into a generic class.

Furthermore, the type of filter necessary for this PR does not seem to exist (the one suggested by this PR is probably quite a hack), and I am unsure what the best course of action in creating such a filter would be. Any advice on that and on how/why synapse must be involved would be appreciated.

Related matrix-js PR for the filter: https://github.com/matrix-org/matrix-js-sdk/pull/2886

Signed-off-by: Arne Wilken [email protected]

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