Change position of "Add" button in Spaces management to make it obvious how to add existing room to space

### Problem

The placement of the `Add` button within the `Explore rooms` page suggests that it functions similarly to the nearby `Remove` and `Mark as not suggested` buttons. The lack of a `Select a room below first` tooltip and a separate button style isn't enough to differentiate the `Add` button from its inline button cousins.

In practice, this results in users (me) failing to add an existing room to a Space. I've forgotten the button's functionality even after using it multiple times :(

### Proposed solution

I've mocked up a different placement for the `Add` button that would (probably) communicate its functionality better.


### Alternative solutions

Remove this Add button entirely, and defer to the `+` buttons beside People and Rooms in the channel tray.

Or if you'd like to go overkill, allow users to drag rooms from the channel tray onto icons in the Spaces tray.

### Additional context

This is a low-priority issue.

```
Element Nightly version: 2021062101
olm version: 3.2.3
```