My brain can't deal with two different ways to write "Tooltip", so this
converges the naming to match the rest of the code base. Separate commits will
fix up the file names for case-insensitive file systems.
* renames RoomTooltip to be a generic Tooltip (which it is)
* hooks it into Field to show validation results
* adds onValidate to Field to let Field instances call an arbitrary validation function
Rebased from @ara4n's https://github.com/matrix-org/matrix-react-sdk/pull/2550
by @jryans. Subsequent commits revise and adapt this work.