added purpose to model (with migration file)
Merge branch 'bulk-imei' into purpose-field-imei-merged

Conflicts:
	src/ralph_assets/models_assets.py
working purpose field in editdevice form
removed migration files
added purpose field to adddevice form (test-passing)
added purpose field to search
added missing form dispatching in Adddevice.post
added field purpose & fixed a name when form data is moving
changed inherited class from (Add|Edit)AssetForm to (Add|Edit)DeviceForm
added migration files
Merge branch 'bulk-imei' into purpose-field-imei-merged
Merge branch 'bulk-imei' into purpose-field-imei-merged
fixed cols matching in 'bulk edit' form & uncommented categories
added purpose field to 'bulk edit' form
fixed tests and minor bugs
make purpose field required when adding or editing device
form_dispatcher is written and used and other code cleanups
increased min-width of bulk inputs
added purpose field to history mechanism & added inting id in history.get_choices(..)
Merge branch 'develop' into purpose-field-imei-merged