Adds edit purpose for plate only available for lab_managers.
Allows to see the purpose from the view of the plate.
Fixes queries for obtaining relevant purposes.
Changes search by a scope. Changes condition
[#74392582] Allow searching by model

This is a cheap speed improvement which might
be useful in the short term, especially when
someone isn't searching for assets.

However the asset search is fundamentally slow
via MySQl so this is not a long term solution.
Merge pull request #885 from JamesGlover/74392582_ss18_-_speed_up_search_(top_right_sequencescape)

[#74392582] Allow searching by model
Merge pull request #876 from emrojo/edit_plate_purpose

Edit plate purpose