feat(*): include additional settings to render emails

- MAILING_ADDRESS
- BASE_URL (was missing from settings)
- SNS_FACEBOOK
- SNS_GOOGLEPLUS
- SNS_TWITTER
feat(common): pass extra context to all emails
feat(common): apply the provided design to the base email template
fix(accounts): redo the welcome email to fulfill proposed content
fix(projects): update Project Apply emails with new content and styles
fix(*): state that BASE_URL must not have a trailing slash
fix(common): remove hardcoded static URL from e-mail template
Merge pull request #113 from brasilcomvc/feat/email-layout

Feature: emails layout