Closes #435.
## Complete redirect rules
- Non-authenticated:
- `LOGIN`: Continue to `LOGIN`
- `ABOUT`: Continue to `ABOUT`
- `PRIVACY_POLICY`: Continue to `PRIVACY_POLICY`
- `EDU`: Continue to `EDU`
- `HOME`: Redirect to `ABOUT`
- Any other page (`destination`): redirect to
`LOGIN?forward=destination`
- Authenticated:
- `LOGIN?forward=destination`: Redirect to `destination`, or `HOME` if
no `destination` is set.
- `ABOUT`: Continue to `ABOUT`
- `PRIVACY_POLICY`: Continue to `PRIVACY_POLICY`
- `EDU`: Continue to `EDU`
- `HOME`: Redirect to `HOME`
- `ONBOARDING`, onboarding needed: Continue to `ONBOARDING`.
- `ONBOARDING?forward=destination`, onboarding not needed: Redirect to
`destination`, or `HOME` if no `destination` is set.
- Any other page (`destination`), onboarding needed: Continue to
`ONBOARDING?forward=destination`.
- Any other page (`destination`), onboarding not needed: Continue to
page (`destination`).
## Scema changes
- In `MagicLoginRequest`, include new `forward` parameter, which
defaults to `HOME`.