[NEW] Option to allow to signup as anonymous

[rodrigok]

@RocketChat/core

Closes #5647
Closes #604

• Rename setting Accounts_AllowAnonymousAccess to Accounts_AllowAnonymousRead
• Add setting Accounts_AllowAnonymousWrite that depends on Accounts_AllowAnonymousRead
• Add a new button to talk as anonymous that creates a new account without email and request a username
• Add a setting Accounts_DefaultUsernamePrefixSuggestion to be able to change the default prefix for username suggestions
• Add a new role anonymous with a few permissions (it's possible to change it at admin area)
• Create the anonymous user with role anonymous
• Change the role to user when user adds and verifies the email adding more permissions

[sampaiodiego]

I'm getting this when showing real names enabled:

[sampaiodiego]

When anonymous write is allowed, guest users can have direct conversations with other users (they cannot only start), but they cannot see the conversations on rooms list, as shown bellow:

[rodrigok]

@sampaiodiego Can you review it again? You'll need to rerun the last migration cuz I did change it.

[sampaiodiego]

I still have found some issues but I dont think they block merging the PR:

• channels created after anonymous user loads the page are not searchable
• after anonymous user joins a channel it doesn't see topic nor description changes until a refresh:
  

[engelgabriel]

Created:
#6817
#6818

[ruKurz]

Does this PR also include anonymous live chat guests. I try to figure out if there is an open issue or PR, that enables live chat guests to start a chat without entering any email address.

[sampaiodiego]

@ruKurz you can already do that disabling pre-registration form:

[ruKurz]

know your product ;-) Thanks for your quick reply!