Enhanced sponsor admin page

For #67:

* admin list sorted by name
* not limited to 100 per page
* a sortable visual indicator for each sponsorship benefit (completed, missing, not applicable)
* also the sponsorship level, admin contact name, and an active or not indication
* the  ones we have today: print logo, web logo, print description, web description and the ad.
* an action pick list like “email” and check mark the sponsors I want to email based on the assets that are missing.
* in subject and body, replace %%NAME%% by sponsor name
Mark for translation and update messages file
Merge branch 'develop' into feature/sponsor-benefits-page

Conflicts:
	locale/fr/LC_MESSAGES/django.po
Merge branch 'feature/sponsor-benefits-page' into develop