Fixes #3812: Only preload selected options for API-based select

[hSaria]

Fixes: #3812

For API-based select fields, only preload the selected options. Other options are dynamically displayed and added via the API when selected.

Example for APISelect

No selection:

<select name="device_role" class="netbox-select2-api form-control select2-hidden-accessible" data-url="/api/dcim/device-roles/" required="" placeholder="Device role" id="id_device_role" tabindex="-1" aria-hidden="true">
  <option value="" selected="">---------</option>
</select>

After selection:

<select name="device_role" class="netbox-select2-api form-control select2-hidden-accessible" data-url="/api/dcim/device-roles/" required="" placeholder="Device role" id="id_device_role" tabindex="-1" aria-hidden="true">
  <option value="" selected="">---------</option>
  <option value="1">Access</option>
</select>

Existing selection:

<select name="device_role" class="netbox-select2-api form-control select2-hidden-accessible" data-url="/api/dcim/device-roles/" required="" placeholder="Device role" id="id_device_role" tabindex="-1" aria-hidden="true">
  <option value="1" selected="">Access</option>
</select>

Example for APISelectMultiple

This is output is from tagged VLANs which may change because of #3589. The list of choices is being manually overridden so that's why they're being created. In reality, the list isn't being used because the API select is dynamically fetching the options and their groups.

No selection:

<select name="tagged_vlans" class="netbox-select2-api form-control select2-hidden-accessible" data-url="/api/ipam/vlans/" data-full="" display-field="display_name" data-multiple="1" placeholder="None" id="id_tagged_vlans" multiple="" tabindex="-1" aria-hidden="true">
  <optgroup label="Global">
  </optgroup>
  <optgroup label="global-group-1">
  </optgroup>
  <optgroup label="London">
  </optgroup>
</select>

Groups are still created if present.

After selection (multiple):

<select name="tagged_vlans" class="netbox-select2-api form-control select2-hidden-accessible" data-url="/api/ipam/vlans/" data-full="" display-field="display_name" data-multiple="1" placeholder="None" id="id_tagged_vlans" multiple="" tabindex="-1" aria-hidden="true">
  <optgroup label="Global">
  </optgroup>
  <optgroup label="global-group-1">
  </optgroup>
  <optgroup label="London">
  </optgroup>
  <option value="1">1 (vlan1)</option>
  <option value="2">2 (vlan2)</option>
</select>

Existing selection (multiple):

<select name="tagged_vlans" class="netbox-select2-api form-control select2-hidden-accessible" data-url="/api/ipam/vlans/" data-full="" display-field="display_name" data-multiple="1" placeholder="None" id="id_tagged_vlans" multiple="" tabindex="-1" aria-hidden="true">
  <optgroup label="Global">
    <option value="2" selected="">2 (vlan2)</option>
  </optgroup>
  <optgroup label="global-group-1">
  </optgroup>
  <optgroup label="London">
    <option value="1" selected="">1 (vlan1)</option>
  </optgroup>
</select>