Make normal user an admin

<!-- Please do not open feature requests for missing parts of the Matrix specification. We are tracking those features under https://github.com/matrix-org/dendrite/issues?q=is%3Aissue+is%3Aopen+label%3Aare-we-synapse-yet --> **Description:** I would like to be able to turn a normal user into a server admin without deleting and re-creating the account. <!-- Describe here the feature you are requesting. -->