Enhancement: Ability to show thumbnails instead of full images on profile pages

[dwanjuki]

Currently Image file-type profile fields are displayed at full-size on the profile pages.
The ability to show a thumbnail(or other image size) that leads to the full image on click, can help make the profile page layouts consistent.

[kimcoleman]

Member directories has its own special function to display a file type preview: https://github.com/strangerstudios/pmpro-member-directory/blob/dev/pmpro-member-directory.php#L127-L143

Core PMPro 3.0+ has a new file type preview version we use in the pmpro_member shortcode: https://github.com/strangerstudios/paid-memberships-pro/blob/v3.0/shortcodes/pmpro_member.php#L179-L189

We should update Member Directory to use the new way for file field preview as it works in v3.0.