Add search and save for the remaining tables

[smcalilly]

Overview

Adds search for the remaining tables. Closes #118.

Other changes include:

• DB migration for LocalAsset model to accommodate for when the asset_id is a charfield instead of an integer. This is required because the PicnicGroves fpd_id is a charfield.
• Another DB migration for LocalAsset model to allow null and blank asset_id fields. This field was meant to save the fpd_uid but in testing this PR, I found an edge case where sometimes that field is null.
  • Also had to create a NullableCharField in the BaseLocalAssetSerializer so we could save this field.
  • The discovery of this edge case led to a discovery of improper 400 error handling when submitting a post request via ajax, so I added better error handling for that.
• Added a popup to the asset geometries in the map.
• For the ParkingLots model, changed the table to search the "acer"."parking_lots_union_mv" and changed the fields to match that materialized view.

Notes

This searches all of the tables that we've discussed with Garret. If we need to fine tune the columns we search, it will be as simple as adding the desired columns to @hancush's clever Search subclass solution. That subclass works in unison with the AssetViewSet'sget_queryset` method.

Testing Instructions

• visit review app: https://ccfp-asset-d-remaining--jg5dia.herokuapp.com/
• login with the ccfp-asset-dashboard test user credentials in lastpass
• go to the asset map tool
• search and save these types of assets:
  • Points of Interest
  • Picnic Groves
  • Parking Lots
  • Signage
• you can test the NullableCharField in the BaseLocalAssetSerializer by searching those asset types with the text "red". that search should return some assets where that field null. you should be able to save the assets when they have a identifier field.

[smcalilly]

@fgregg this is ready for re-review. you can test the review app at this new instance: https://ccfp-asset-d-remaining--jg5dia.herokuapp.com/