How do I bulk rename images and videos in photos?

You can't change the file name of a picture, but you can add a title-- it's the first field in the Info window. If you click on on a bunch of pictures, you can give them all the same title by typing it into the title field.

When I want to add indexing or different dates to pictures, I use the app Photos Workbench ($30). It lets you set up a number of different sequences using a range of specifications. Here is an example:

Or this:

Here is a partial list of choices you have for inclusion in the title:

The app also allows you to edit titles with find and replace. So you can change all the tiles that have "Playground" in them so the they say "Jungle Gym" instead. I has a number of other cool features, as well.

You can use this AppleScript to re-Title photos and videos in a Photos library:

Batch Change Title to Capture Date-Description-Padded Sequential Numbers

For a limited time, until June 13th, the script compiled as an application can be downloaded from this tutorial site:  P01 - Applescripts from Photos’ User Tips Compiled as Applications  

When you need the photos outside of the library export using the Title as the new file name.