added className and text properties to DataTable to style export button

[jwbargsten]

This PR adds two props for the DataTable, export_className and export_text. This allows to modify the class and the text of the export button. Very useful if you want to style the button with e.g. bootstrap classes or translate the button text.

Pre-Merge checklist

• [ ] All tests on CircleCI have passed.
• [ ] All visual regression differences have been approved.
• [x] You have added an entry describing the change at the the top of CHANGELOG.md. The entry should also link to the original pull request(s).
• [ ] Two Dash core contributors have :dancer:'d the pull request.

Post-Merge checklist

• [ ] You have deleted the branch.
• [ ] You have closed all issues that this pull request solves.
• [ ] If significant enough, you have created an issue about documenting the new feature or change and you have added it to the dash-docs project.

[bratao]

Any chance of approving this MR? @alexcjohnson For App in other languages this MR would be essential

[alexcjohnson]

@jwbargsten @bratao sorry, somehow this one slipped off our radar. It looks good, I just had a couple of minor comments.

[jwbargsten]

something went wrong during rebase on dev. I'll fix it and then wrap it up.

[jwbargsten]

I rebased/fixed this PR now 2x, and given the lack of progress, I somehow lost (a little bit) the confidence that this will be resolved. I suggest to close it therefore.