Datastore char encoding

[beeyayjay]

fixes #2901

Description. Adds ability to add non UTF-8 files to datastore.

User story

Currently datastore breaks if the file isn't in UTF-8 .
This adds a select list of allowed PHP encodings to the datastore form. A user can choose the appropriate encoding for the file.

QA Steps

1. Create a new resource, uploading /dkan/test/phpunit/dkan_datastore/data/win1252_encoding.csv.
2. Note that the data preview displays bad characters in the 'title' field
3. Go to 'Manage Datastore' and select 'Windows-1252' as the Character encoding.
4. Click 'Import'.
5. Confirm that the file imports successfully and the special characters now display properly in data preview