Delete an organisation member

[nid90]

Closes: #278

Duplicate and updated version of #668

[samrat]

I ran into two errors when opening the page as a user who is not part of any organization:

• page_title was erroring out
• Organization dropdown in header was failing to render.

I've pushed a commit to fix the two issues.

This is what the "no organizations" page looks like now(title is just "Tramline" and org dropdown hidden):

[samrat]

Made invitations cancellable. When a user visits a discarded invite link they'll see this error:

[nid90]

@samrat I think we should handle the invalid invite token a little better. There are two places where the invite acceptance flow can go - Authentication::Email::ConfirmationsController for existing users and Authentication::Email::RegistrationsController for new users. We should show proper messages when the invite token is no longer valid.

[samrat]

Improved the error messages.

When unregistered email visits discarded invite link

When registered email visits discarded invite link

• redirects to root path with a flash message

[nid90]

@samrat lgtm. merging.

made some minor UI changes. we use EmptyStateComponent for all empty state pages/sections. also, added handling when the invite is not found. as a general rule, we avoid find_by! or find in user-facing actions as that will blow up with a 500 for the user.