Allow configuration of close, cancel buttons on Dialog

[lyzadanger]

This PR contains updates to Dialog to address a couple of application use cases dealing with close and cancel buttons.

Part of hypothesis/lms#3139
Related to / a dependency for hypothesis/lms#3106

Dialog with a close button but no cancel button

I want to be able to replace the SidebarPanel Panel components in the client with Dialog for consistency and the nice focus routing that Dialog provides:

That UI renders a close button but does not render any buttons in the panel/dialog itself. Previous to these changes, the Dialog component would always automatically render both a close and a cancel button if an onCancel (i.e. close callback) is provided. Now there is granular control via two new props: withCloseButton and withCancelButton (which default to true such that default behavior is unchanged).

Similarly, a modal with a cancel button but no close button

In LMS, the OAuth2RedirectErrorApp modals have a cancel button (it's labeled "Close" here but it is a "cancel button" from the perspective of the Dialog) but no close ("X") button:

These changes support this.

[codecov]

Codecov Report

Merging #203 (d889191) into main (2cf9a01) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main      #203   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           14        14           
  Lines          229       236    +7     
  Branches        80        87    +7     
=========================================
+ Hits           229       236    +7     

Impacted Files	Coverage Δ
src/components/Dialog.js	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 2cf9a01...d889191. Read the comment docs.

[robertknight]

LGTM