Add Generic Webhooks Widget interface

[Half-Shot]

This PR improves the general interface around admin room controls for widgets. The outcome will hopefully be mapping the complete set of configuration options and tools to the widget web app.

Previously, this PR demoed an experiment to show invite widgets (that eventually became the Slack invite widget experiment). However, invite UI in hookshot isn't particularly useful so it was removed from this PR. This PR now does other things.

• When inviting hookshot into a room, it will optionally spawn a room configuration widget.
• The room configuration widget can be used to create, update and delete generic widgets.
• There is a lot of room in the code for other service types to be added, but this PR is constrained to just webhooks for now.
• This PR also refactors the widget API to use matrix-appservice-bridge's ProvisioningAPI.
• This now means that widgets no longer use the archaic access-token-stored-in-state logic we had in admin rooms, instead using short lived tokens.

Video

TODO:

• Documentation

[HarHarLinks]

video demo looks quite excellent.

Q: (how) do you prevent leaking the webhook url to anyone in the room besides the admin? can't anyone use the widget? => is this what the admin room setting is supposed to solve?

[Half-Shot]

video demo looks quite excellent.

Q: (how) do you prevent leaking the webhook url to anyone in the room besides the admin? can't anyone use the widget? => is this what the admin room setting is supposed to solve?

URLs are "secret", so non-admins basically can see the code and the name (i.e. what you can see from the state event).

[tadzik]

Admittedly, I've only skimmed the React bits – having them in scope of all this feels a bit overwhelming, but I'm not sure if it makes practical sense to separate it.