Webhooks bridge support

[turt2live]

Fixes #55

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This serves as a good example for what to do in Dimension to add a bridge. There's really only 3 commits that need to be referenced to get a bridge working:

• 93b532d - This adds the bridge interaction layer and has no UI. This is what Dimension will be using to actually talk to the bridge itself.
• 7b5285c - This is the admin section of the bridge. Here administrators will configure whatever is needed for Dimension to talk to the bridge. In most cases this will be the URL and shared secret to use, however other options may also be included here such as features to enable or disable. Bridges with an upstream for matrix.org are a bit complicated - take a look at the IRC code in the same area as a reference.
• 3823788 - This is the last bit of the bridge support. This commit adds the required UI so that end users can configure the bridge for their room.