added *imei* field to model & search && edit form (only in BO) + validatio
better var name
Merge branch 'develop' into imei-field-ver2
moved imei from OfficeForm to Base(Add|Edit)AssetForm
moved *imei* field to Base(Add|Edit)AssetForm (instead of OfficeInfo)
added category-imei fields dependency
deleted code cleanups
some correction by pylint
fixed sidebar caption
added *category* field to *bulk edit* form + minor bug fixes
added *imei* field to bulkedit
code cleanups
saving imei in AddDevice & AddPart forms
fixed tests
docstring corrected
code cleanups
model imei must be unique, imei migration file deleted
Merge branch 'develop' into imei-field-ver2

Conflicts:
	src/ralph_assets/forms.py
	src/ralph_assets/views.py
removed fields *category* *imei* from bulk
added migration file for imei