Allow anonymous admins to use admin scoped commands

[rayz1065]

Currently anonymous admins are unable to use admin scoped commands, despite them being displayed in the user interface. The docs say about sender_chat:

Optional. Sender of the message when sent on behalf of a chat. For example, the supergroup itself for messages sent by its anonymous administrators or a linked channel for messages automatically forwarded to the channel's discussion group. [...]

There should be no other way to send a message on behalf of a group.

This pull request updates the isAdmin function to take into account anonymous admins.

[roziscoding]

@carafelix can you please TAL at this?

[carafelix]

Really glad to see this PR. A well thought addition.

Requested changes are mostly about unused lines.

I think lines 1030-1032 need an explanation or change.

Thanks for checking out on this.

[carafelix]

LGTM, thanks for the further explanations