Refactor import functionality

[orenyk]

We currently have a lot of duplicated code between our import equipment functionality (#494) and our import users functionality (#6), this should be abstracted out into a generic import library that can be applied to various models / resources. See #897 for @dgoerger's comments as well.

[orenyk]

This should include tests for both generic import functionality as well as user and equipment imports specifically.

[orenyk]

We also need to write controller tests for all of the import controllers (see #404).

[orenyk]

Note that we should have a general ImportController and more straightforward routes (/import/users and /import/equipment, see #1141)

[orenyk]

I think we'll do most of this as a module that will be included in the relevant classes; we'll have methods for checking the CSV keys and other stuff, as well as methods for returning the correct error messages. One change we should make is ideally not enforcing the order of CSV columns. Since we're simply assigning attributes using a Hash, we shouldn't care about order (although we can validate that all column headings are unique).

[esoterik]

This should also include importing a combined import/export page, as discussed in #1340