Fixes (#435)

* Add flight level field to PIREP field closes #401

* Default value for distance 0 closes #400

* Block airline deletion if assets exist #367

* Formatting

* Move some of the base exception classes

* Fix skin references to use settings table

* Set default for theme name if setting is wrong

app/Models/Subfleet.php

@@ -18,6 +18,7 @@ use App\Models\Traits\FilesTrait;
  * @property float   cost_block_hour
  * @property float   cost_delay_minute
  * @property Airline airline
+ * @property int     airline_id
  */
 class Subfleet extends Model
 {