Disable URL previews for internal message links

[jryans]

Some Riot installations (including those on Modular, like Mozilla's) are configured via permalinkPrefix to use Riot's origin as the prefix for message permalinks.

When you then post such links in a room with URL previewing, Riot dutifully adds a silly preview of itself with the Riot app icon, which has nothing to do with the message.

In some ideal world, we'd actually preview the target message, but for now we should at least disable previewing when pointing at ourselves.

[MadLittleMods]

Related to matrix-org/matrix.to#182

[turt2live]

ftr, as an opposite to this: there's specific sponsored requests to in fact make the preview richer instead (akin to Discord)