Replace "Administrator" with "Owner" of a room to make the consequence of co-ownership more obvious

#### Is your suggestion related to a problem? Please describe.
Losing sovereignty on a room can happen since people tend to grant Administrator PL to others quite easily. It's seen as a technical role, but the "room governance" aspects are often overlooked by people who grant others administrators.

#### Describe the solution you'd like.
I looked up in the Matrix protocol specification, and it seems to only mention power levels but doesn't really give it name. [It only suggests the following](https://matrix.org/docs/spec/client_server/latest#permissions):

> Clients may wish to assign names to particular power levels. A suggested mapping is as follows: - 0 User - 50 Moderator - 100 Admin

When talking of the "owner" of a room (instead of administrator), and "sharing ownership" (instead of granting admin privileges), people have a more intuitive understanding over the fact that they can't take it back.

#### Additional context
NeoChat already uses the Owner label for people with PL 100