Add Jinja2 support for export templates

[jeremystretch]

Environment

• Python version: 3.5.6
• NetBox version: 2.5.9

Proposed Functionality

NetBox allows users to create arbitrary export templates for objects. Currently, these templates must be written in Django's native templating language. This request seeks to add Jinja2 as an alternate template language. While Jinja2 and Django's templating languae are very similar, Jinja2 has the advantages of being more flexible and more commonly understood.

Use Case

Jinja2 allows for more complex language within templates, whereas Django's template language is intentionally limited (with the design goal of pushing logic into the view component of Django's MVT framework).

Database Changes

Add a "template language" field to the ExportTemplate model. This will be set to Django for all existing templates but will default to Jinja2 for all new templates.

External Dependencies

Jinja2 is currently pulled in as a dependency by drf-yasg. It will be listed in requirements.txt as a top-level dependency but should not have any noticeable effect on existing installations.

[Grokzen]

@jeremystretch This feature really needs to have docs added to it here https://github.com/digitalocean/netbox/edit/master/docs/additional-features/export-templates.md