Add description/title to Image selection sheets

[iamthomasbishop]

This won't be as big of an issue when we change the pencil (replace) icon eventually, but in the meantime, we should add some descriptive titles to the top of the bottom sheet, for some added clarity in the flow.

Replacing an Image:

For each platform, I'm hoping we can:

• iOS: add as a top row description to the ActionSheet
• Android: add it as a standard list item – text label color is $gray-darken-10 instead of $gray-dark

This could also apply to initial image selection:

[hypest]

Removing the milestone marker since we nowadays only add PRs to milestones.

[SergioEstevao]

@iamthomasbishop do you think the new icon for replacing image is enough? Or do you think we still need to add the extra information on the top of the action sheets?

[iamthomasbishop]

We should probably have a description at the top of the sheet, as I don’t think the icon is always sufficiently clear.