Little bit of a mix of things in this one:
* Support variable-width dialogs. Default is fixed-width as before,
only UploadConformDialog is variable-width. Controlled by a prop
to BaseDialog.
* Fixes to the cancel 'x' - scale the mask image, tweak size & colour
* Colour & boldness of dialog titles
* Align the dialog title & cancel 'x'
* Remove gap between dialog buttons & right hand side of dialog(!)
* Round corners on dialogs
* Add grey border on image preview in upload confirm dialog
* and, squeezing in slightly randomly, finish the partially renamed
ChatInviteDialog to AddressPickerDialog.
* 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.
If the dark theme is activated, the country dropdown defaults to those colors
where it doesn't fit in with the always-light auth flow. This restyles its
colors to be light.
