Add an 'Add Existing User' button to collections in Security and Accounts

[lexiclevenger]

Is your feature request related to a problem? Please describe.
Currently, to assign an existing account to another collection, the user must edit the account and change the Collection pick list value. This behavior is not intuitive because it suggests that the account is being removed from the previous collection.

Describe the solution you'd like
An Add Existing User button should be added to the Collection panel in Security and Accounts-- a similar feature exists in Specify 6.