Adds sending of email and design changes

### Description

- Added email for cases mentioned in #https://github.com/pythonindia/pssi.org.in/issues/39. Fixes #39.
- Removed Django signals
- `emailer` file in `common` app contains email sending functionality. As of now we don't need multipart email.

#### Sending email on update

- As of now admin has to explicitly select the row from interface and select action to send email. This method will prevent sending emails multiple times
by different staffs.

#### After deployment

- Add `GrantType` via Django Admin.
- Add `MEMBERSHIP_PAYMENT_LINK` (Instamojo url) in settings file before going live.

@iambibhas code review ?