Emit user notifications from chain events.

[jnaviask]

Description

This PR tracks the progress of the "chain notifications" project. The intent is to fetch events from various chains, process them on the server, possibly storing them in the database, and emit them as notifications to the end user.

Motivation and Context

We already have a notification system in place, but it currently provides notifications for discussion events only. We would like to expand its scope and notify users about other important events taking place on chains they're interested in.

How Has This Been Tested?

Unit test cases and running it with a local edgeware chain.

TODO for MVP

• Remove manage_chain_notifications_modal and add subscription types to the /subscriptions page.
• Add proper formatting for displaying balances within notifications.
• (is this possible?) Ensure that proposal event creators don't receive a notification immediately upon creation.
• Add webhook data.
  • Separate PR.
• @raykyri Ensure interface of notification pulldown is correct -- I moved some buttons around, we should confirm their positioning.
• @raykyri Verify that url-linking when clicking on a notification is working as intended in all cases (e.g. if the proposal page no longer exists, or if we have no link available).
• @raykyri Verify subscriptions page looks OK.

Types of changes

• New feature (non-breaking change which adds functionality)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• My change should be included in the release notes.
• I have updated the documentation accordingly.
• I have linted the code locally prior to submission.