Refactor settings files

The current way settings work, they set a bunch of things and
then try to import local_settings.py, which means local_settings.py
cannot modify anything set in the importing file; it has to
completely override it.

These changes make local_settings.py the top-level file, which needs
to be created before anything will work. It should import any of a
number of environment-specific settings files (dev, staging, test,
production) and can then override anything it needs to.
DB settings in base; move local with others

* Add default database settings to pycon/settings/base.py
* Move local settings to pycon/settings/local.py
Merge branch 'develop' into feature/78-refactor-settings
Update settings to get server values from environment
Merge branch 'develop' into feature/78-refactor-settings
Bool in env will be 'true' or 'false'
Merge branch 'develop' into feature/78-refactor-settings
adding speakers as csv string to email template context
Sending email to all speakers per proposal
Adding eg for speakers in email notification template
remove adding to_address, as the primary speaker is already a member of self.proposal.speakers()
Merge branch 'develop' into feature/77-co-speakers-email-notifications
Merge branch 'develop' into feature/78-refactor-settings
Cleanup, comments, tweaks
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
Doc and comments
Rename README to README.rst
README formatting
Use settings from env by default in base settings
Tweak docs based on testing