Nonfunctional email alert form

[alexander-griffen]

Addition of email alert form into web app.

This involved:

• Adding 'region' and 'country' as fields to the Council models
• Moving filters to a separate file, 'filters.py'
• Creating the view + template for the email alerts page
• Adding to the TenderFilter to create necessary filters for the form
• Creating a new file for autocompleting just the councils, without redirecting them
• Tests for email alerts form.

There is still some frontend jankiness, regarding selecting all councils/a region/ a specific council, which hasn't yet been solved.

[struan]

I don't think the council selection bit is working correctly, in that if I select my council, fill in a council and submit, and then select councils in a region and select a region it tries to apply both.