Enhancement/redirect to welcome page

[henriquecbuss]

What issue does this PR close

Closes #571

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

• Add redirect query parameter to /community/selector, so that users can change communities and stay on the same page
• Community selector modal now redirects to the same page the user is in, but in another community
• If a Guest tries to access a community that doesn't exist, they're redirected to the welcome page (https://www.cambiatus.com/welcome or https://www.cambiatus.com/welcome-demo)

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

Testing the redirection on /community/selector:

1. Log in to a community
2. Now try going to a community that doesn't exist, in a specific page (e.g. https://thisisntacommunity.staging.cambiatus.io/shop)
3. You should be redirected to /community/selector
4. Choose a community
5. You should land in the community you chose, in the page you chose (/shop)

Testing the redirection on the community selector modal:

1. Log in to a community
2. Go to some page (e.g. /shop)
3. Click on the community selector icon on the top left
4. Choose another community
5. Be redirected to that community, on the page you chose (/shop)

Testing the redirection to the welcome page:
Since this depends on the URL, we could only test this if it was deployed on demo 🙁, so look through the code and see if it makes sense

[lucca65]

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