Alerts for job posts based on filtersets [WIP]

[shreyas-satish]

• Adds the ability to subscribe to alerts for job posts via email. The alerts can be configured for job posts that match one or multiple combinations for filters. Support for push notifications will be added in the next release.
• Adds a custom __init__ for Filterset to help create filtersets using a dict of filters.

Job alert email

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:white_check_mark: vidya-ram
:x: iambibhas
:x: shreyas-satish
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