Code cleanup/improvements

[Morsey187]

Some minor code improvements not yet implemented from a review of the initial commit:

1. Delete media/.gitignore and use root .gitignore instead.
   Note: code was copied from bakerydemo, although approach is likely not necassary.

2. Pass request object when calling .url on a page to avoid performance issues (see LinkStructValue class).

3. Improve handling of placeholder image.
   get_placeholder_image shouldn't try and provide a fallback image or should at least point to a static file instead.

4. Remove querystring_modify, perhaps with a tag from wagtailadmin_tags if possible.

5. Update responsive images, Resolution switching is being used for srcset, although the same values are being used across different display resolutions (1x vs 2x).

[zerolab]

re: querystring_modify, https://github.com/ababic/django-querystring-tag/ may be a better alternative