Part page loading improvements (#3185)

* Lazy load the pricing bom table when the "pricing" tab is selected

* Update django-debug-toolbar configuration

* Major refactoring for the 'can_build' function

- Use a single annotated query to the db, rather than a for loop (which is what a caveman would use)
- Query performance is greatly improved
- Also refactors existing variant-part-stock subquery code, to make it re-usable

* Use minified JS and CSS where possible

* Render a 'preview' version of each part image

- Saves load time when the image is quite large
- Adds a data migration to render out the new variation

* Adds 'preview' version of company images

* Defer loading of javascript files

Note: some cannot be deferred - jquery in particular

* Crucial bugfix for user roles context

- Previously was *not* being calculated correctly
- A non-superuser role would most likely display pages incorrectly

* Prevent loading of "about" on every page

- Load dynamically when requested
- Takes ~400ms!
- Cuts out a lot of fat

* Match displayed image size to preview image size

* Utilize caching framework for accessing user "role" information

- Reduces number of DB queries required by rendering framework

* Remove redundant query elements

* Remove 'stock' field from PartBrief serializer

- A calculated field on a serializer is a *bad idea* when that calculation requires a DB hit

* Query improvements for StockItem serializer

- Remove calculated fields
- Fix annotations

* Bug fixes

* Remove JS load test

- Loading of JS files is now deferred, so the unit test does not work as it used to

* Fix broken template for "maintenance" page

* Remove thumbnail generation migrations

- Already performed manually as part of ''invoke migrate"
- Running as a migration causes unit test problems
- Not sensible to run this as a data-migration anyway

* tweak for build table

InvenTree/build/templates/build/build_base.html

@@ -17,7 +17,7 @@
 {% block thumbnail %}
 <img class="part-thumb"
 {% if build.part.image %}
-src="{{ build.part.image.url }}"
+src="{{ build.part.image.preview.url }}"
 {% else %}
 src="{% static 'img/blank_image.png' %}"
 {% endif %}/>