Ensure every UserAccount always has an associated User

[mattyg]

In #359 we noted that it would be great to be able to track a person's interaction with the praise system over time.

Currently we can do so with the useraccount._id. A useraccount represents a person's account on a 3rd party platform. Currently the only platform available is Discord, so we can simply include the useraccount._id in the exported data and be able to analyze their activity over time.

However when we add another platform (Telegram), each person could have 2 useraccounts: one on Discord, and one on Telegram. Now it is not enought to include the useraccount._id to track a person's interactions over time, since it only represents their interactions on one platform.

Further there is a lot of redundant logic throughout the codebase simply to display the "name" of the user / useraccount.

Overview

• Create an "inactive" user for each new (unactivated) useraccount. Thus the useraccount will always be associated with a user.
• When the person activates their useraccount, it simply makes the associated user active.
• If a person has multiple useraccounts associated with different users, they will be merged with the same user when activated from the same ethereum address. Thus one of the users will be "orphaned" (with no associated useraccount). All references to that user will need to be updated to the newly active user, and the orphaned user will be deleted.
• Refactor UserAccount component
• Remove getUsername helper utility and refactor all components using it.
• Refactor eventlog to only store the user (not the useraccount), migration for past eventlogs to reflect the new data format