Use IconButton for nicer "edit" and "remove revision" icons, and set default appearance for all buttons

[julienw]

It's probably easier to look at commits separately.

• Commit 1: uses IconButton instead of Button for some of our icons -- namely the "edit" button and the "remove revision" button. This makes these buttons much nicer in the process, especially on hover. Note that this was needed for setting the variant by default in commit 2, that's why I included it in this PR too.

• Commit 2: set some defaults for all buttons: variant = contained so that they look like button, and disableElevation to remove box shadows. I think that the shared styles do some of "variant = contained" too and we might want to simplify them later, but I didn't want to do that now. (see https://mui.com/material-ui/customization/theme-components/#theme-default-props about this -- we could also add disableRipple if we want to)

• Commit 3: remove the "variant" property where appropriate now that it's set globally.

• Commit 4: update test snapshots

To summarize visible changes:

• removes the box shadows on buttons like the design says
• makes the "remove revision" icon a bit bigger
• makes the hover style for both the "edit" icon and "remove revision" icon nicer, and their hit size bigger
• Some spacings are slightly different too but I think I managed to keep these changes minimal.

I think that the only debatable change is making the variant contained by default. It can still be overriden on a case by case basis by using variant="text" or "outlined".

Please tell me what you think!

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.18%. Comparing base (ce5919d) to head (8a4753e).

❗ Current head 8a4753e differs from pull request most recent head 44b7926

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@@           Coverage Diff           @@
##             beta     #665   +/-   ##
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  Coverage   97.17%   97.18%           
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  Files          67       67           
  Lines        1488     1491    +3     
  Branches      256      257    +1     
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+ Hits         1446     1449    +3     
  Misses         38       38           
  Partials        4        4           

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[Carla-Moz]

Thanks the buttons styles cleanup! Along with removing the box shadows.