Satellite signup: Update activation flow

[mobyvb]

Implement a new email verification method that makes use of a 6-digit emailed code rather than an activation link.

• after signup, the user is directed to a page requesting to enter a 6-digit code
• the user receives an email containing the code, which can be copy-pasted into the UI to activate the email
• the activation email does not contain a link that can be clicked

Because this feature makes significant changes to common workflows, we should implement it with a feature flag, so that we can easily switch between link-based email verification and one-time-code-based email verification.

AC:

• backend:
  • add a new type of email for verifying email that contains a code to be inputted into the UI, and no activation link
  • when a new account is created, store a random 6-digit code associated with that user in the DB, and send an email with that code
• frontend:
  • implement the verification code in Vuetify (this is already a part of existing Vuetify mockups)
  • implement the verification code in the production app (can be templated off of the Vuetify version)
• misc:
  • add a feature flag that allows us to use either link-based email verification, or code-based email verification. If using link-based email verification, it should behave the same as before
  • make sure it is possible to create an active, verified account in test setups (e.g. storj-up, storj-sim) where emails cannot be easily accessed

[storj-gerrit]

Change satellite/db: add activation_code column mentions this issue.

[storj-gerrit]

Change satellite/{console,web}: add activation code config flag mentions this issue.

[storj-gerrit]

Change satellite/{console,emails}: add activation code email mentions this issue.

[storj-gerrit]

Change satellite/console: implement account activation with code mentions this issue.

[storj-gerrit]

Change web/satellite: implement account activation with code mentions this issue.