Fix Button with display="inline" to use the same styles as the regular button

[lindapaiste]

Fixes #2821

This does not change any UI anywhere in the app. It removes some unused code and enables us to use the <Button> component inline in the future. There is more that could be refactored and cleaned up but I'm keeping this PR minimal.

Changes:

• Modify the core StyledButton component to use either display: inline-flex or display: flex, depending on the value of the display prop.
• Deleted the StyledIconButton component which was not used anywhere. Confusingly, the existing IconButton component which we use on mobile is an extension of the StyledInlineButton rather than the StyledIconButton.
• Maintain the styles of the current IconButton component by passing down the prop iconOnly and using this prop to switch components rather than switching based on the display prop.

I have verified that this pull request:

• has no linting errors (npm run lint)
• has no test errors (npm run test)
• is from a uniquely-named feature branch and is up to date with the develop branch.
• is descriptively named and links to an issue number, i.e. Fixes #123

[PiyushChandra17]

Nicely Done 👍

[raclim]

I think this looks good to me! Thanks for working on this!