audit all the current fields of SiteSettings

[mhl]

Three fields which are currently in SiteSettings shouldn't be - they're used by core Django, so we can't easily modify that code to use a value from SiteSettings instead:

• DEFAULT_FROM_EMAIL
• SERVER_EMAIL
• DATE_FORMAT

These should be removed from SiteSettings.

The following remaining settings are OK to handle as values of SiteSettings, but I've spotted many instances in the code when their value from django.conf.settings is used instead of SiteSettings - these all need to be checked, and any wrong ones fixed:

• SITE_OWNER
• SITE_OWNER_URL
• COPYRIGHT_HOLDER
• TWITTER_USERNAME
• SUPPORT_EMAIL
• DD_MM_DATE_FORMAT_PREFERRED
• MAPIT_BASE_URL
• GOOGLE_ANALYTICS_ACCOUNT
• USE_UNIVERSAL_ANALYTICS
• NEW_ACCOUNTS_ALLOWED
• HOIST_ELECTED_CANDIDATES
• RESTRICT_RENAMES
• EDITS_ALLOWED
• CANDIDATES_REQUIRED_FOR_WEIGHTED_PARTY_LIST
• TWITTER_APP_ONLY_BEARER_TOKEN
• IMAGE_PROXY_URL