add missing fixDialogButtons to signup-dialog
add missing fixDialogButtons to signup-dialog

refs FOO-1869

Fix flakey `users registration should require terms if configured to do so` by adding a missing fixDialogButtons import. Currently, this gets imported via runOnEveryPageButDontBlockAnythingElse.js, so sometimes it doesn’t exist yet when this code is run.

Change-Id: I8d191109486f69ed776a09eb08e78983b831dcbf
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/263638
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
Reviewed-by: Ahmad Amireh <a53a33601b8dd9d06ae9e50f1f30fbe957aba866@instructure.com>
QA-Review: Aaron Ogata <11ccf682c06d1508642f943a3962a1c3d00b8e44@instructure.com>
Product-Review: Aaron Ogata <11ccf682c06d1508642f943a3962a1c3d00b8e44@instructure.com>

instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms