Use getTextColor for retention heatmap cells so text is visible on dark backgrounds

[rgrunberg]

The retention heatmap cell text is difficult to read on a dark background. This PR:

• Pulls getTextColor out of the conditional formatting functions and make it a general shared function
• Colors the text of the retention heatmap cells according to the background shade

Before:

After:

[rgrunberg]

Hi @Jonrosario5 are you able to approve PRs? Or do you have advice on getting PRs merged in this repo? Thanks!

[Jonrosario5]

Hi @rgrunberg I don't have access to approve the PR because it is protected and only specific admins can merge into this Repo. But I'll notify our team and will have someone review the merge and approve it.

[rgrunberg]

@Jonrosario5 thank you!