Fix/join with invitation

[henriquecbuss]

What issue does this PR close

Closes #590

Changes Proposed ( a list of new changes introduced by this PR)

• After a user signs up through an invite link, redirect them directly to the dashboard instead of back to the invite page
• Make it easier to have a page that can be accessed either by a logged in user or a guest user
• Prevent user being redirected to the community selector when they follow an invitation link

How to test ( a list of instructions on how to test this PR)

1. Log into a community
2. Follow an invite link for a community that only allows for invited users and that you're not a part of (you can use http://test.staging.cambiatus.io/invite/Zrdow9)
3. See invitation page, where you can choose to join the community, in which case you should be redirected to the dashboard

[lucca65]

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