Cleanup main view and add tests

[chosak]

This commit converts the landing page (main view) into a Django class-based view and moves it into its own module. It also adds various unit tests to cover this view.

This is a partial fix to the issue in #44 where non-GET (e.g. POST) requests to this view would raise a 500 error.

This commit adds a dummy data/header-only.xml RegML file (which validates against the schema) that can be used for simple testing of this view only. The new unit tests for this view validate that it works with no data or with data loaded.

This commit also adds a simple unittest against eregs_core.urls to ensure that the URLs module can be imported (to protect against syntax errors which would otherwise not be discovered via the test suite).

Additions

• New test cases against the main view and the urls.

Changes

• Moves eregs_core.views.main into a Django class-based view at eregs_core.views.main.MainView.

Testing

• Run unit tests with e.g. tox -e dj18.
• Run a local server with ./manage.py runserver and navigate to http://localhost:8000.

Checklist

• Changes are limited to a single goal (no scope creep)
• Code can be automatically merged (no conflicts)
• Code follows the standards laid out in the front end playbook
• Passes all existing automated tests
• New functions include new tests
• Visually tested in supported browsers and devices
• Project documentation has been updated (including the "Unreleased" section of the CHANGELOG)

[grapesmoker]

Just curious, what's the reason for this? I get that posting to main raises a 500, but... you shouldn't be allowed to post there. We can add a case to handle that, too. Does the class-based view provide us with some additional flexibility?

[chosak]

CBVs make views easier to test and reduce code duplication. Using CBVs automatically provides proper handling of non-GET requests (returning 405), meaning we don't have to reimplement it for every view. We could implement "you shouldn't be allowed to POST there" by putting logic like

if request.method != 'GET':
    return HttpResponseNotAllowed()

everywhere it's needed but then we'd have to reproduce this on every single view (see #44).

Similarly CBVs let us use standard Django patterns like defining template_name on the class instead of having to manually call render_to_response.

[grapesmoker]

Cool, that makes sense.