IA2: Concept

As a part of [Phase II of IA](https://github.com/vector-im/element-meta/issues/56), in low fidelity, we'd like to gain confidence in how to build out Discovery or Build efforts for:

- [ ] https://github.com/vector-im/element-web/issues/20614
- [x] https://github.com/vector-im/element-web/issues/20616
- [x] https://github.com/vector-im/element-web/issues/20618
- [ ] https://github.com/vector-im/element-web/issues/20629

__________

Relevant notes from original IA epic:

* 2.1: Room header updates
* Update the icons in the top right so every icon is a discrete interaction launched/exited discretely
* Show expanded topic when clicking on the topic in the room header
* Update overflow iconography to trigger right panel
* 2.2: Search [[Figma](https://www.figma.com/file/u6xYQamXT4BqUJ5cKnkbWr/iA-Prop?node-id=491%3A77978)]
* Expand search spotlight experience with tabs for a unified search experience.
* Rooms you've joined
* Rooms within Space
* People
* Messages
* etc
* Remove search icon from room header?
* 2.3: Explore Space [[Figma](https://www.figma.com/file/xQ38nQnOmMSIlhYEL3sm6P/Milestone-2%3A-early-work?node-id=131%3A4214)]
* Refresh Explore to aid discovering and browsing rooms
* Add 'Join all' interaction for suggested rooms
* 2.4: Preferences [[Figma](https://www.figma.com/file/u6xYQamXT4BqUJ5cKnkbWr/iA-Prop?node-id=508%3A83448)]
* Let users customise look/feel and sections either per space or per account
* 2.5: Subspaces [[Figma](https://www.figma.com/file/u6xYQamXT4BqUJ5cKnkbWr/iA-Prop?node-id=650%3A134867)]
* Labs: Move Subspaces into room list, as sections or column interactions, informed by subspace membership.
* Reduce friction to join/leave subspaces in Explore Space, and room list interactions.
* Add ability to pin Subspaces from the room list to the space panel.