Admin features

**Admin resources page:** 
+ Added functional “Expand All” and “Collapse All” buttons to admin resources page.
+ Added functional filtering feature to admin resources page.

**Back-end:**
+ Added new booleans to `FoodResource` model.
+ Added analogous booleans to `AddNewResourceForm`.
+ Added / Refined `serialize` functions for various models.

**Map:**
+ Styled the map using CSS.
+ Updated pin images that are used by map (non-outlined to outlined.) 
+ Added filtering checkboxes to map front-end (not functional yet.) 
I added a filtering system to the top of the admin resources page that allows an admin to view a subset of all of the food resources based upon a few filtering criteria (e.g., zip code, is-for-seniors, is-for-family-and-children, etc.) This seemed like a good feature to implement given that there will be hundreds of resources on this page eventually, and examining them all together might be kind of unwieldy. It was also a good opportunity to understand how filtering can be implemented, as we'll have to do something very similar for the map page. 

This particular line of code fires when the user clicks on the "Apply Filter" button on the admin resources page. 

Does that make sense? Do you have feedback on that feature? If you want to try it out yourself, you might consider pulling from the `admin-features` branch onto a new local branch.