Add support for i18n via --locale

[satyamtg]

Fixes #148 by adding support for i18n and RTL layout. The following changes have been made at large -

• main.css and thread.css have two versions now, one for RTL, other for LTR
• LANGUAGE_COOKIES defined in constants.py are now set to retrieve assets in proper language and layout
• The new parameter --locale is introduced to support i18n
• Templates are tweaked to have support for RTL (the RTL presence is detected while scraping the homepage)
• Translations for Hindi language are added. However, there's very less to translate as most of the content is translated already and some which are left out, are fetched properly after setting correct value of the cookie.
• OPENEDX_LANG_MAP is introduced in constants.py to map our iso-639-1 codes to the codes used by openedx (these values might need updating in the long run)

[stale[bot]]

This pull request has been automatically marked as stale because it has not had recent activity. It will be now be reviewed manually. Thank you for your contributions.