Name and order the Telegram bot code

Name and order the Telegram bot code

Every package-level name of the Telegram bot starts with telegram, and
its files start with the same name: the Bot API client is in
telegram_api.go and the Markdown conversion in telegram_markdown.go. So
the names of the bot do not read as names of the model client or of the
login, which also have a client, an API, and a poll. Type names start
with a capital letter, so that a variable can have the name of its type.

The declarations of the Telegram files follow the order of the go-code
skill: entry points, then each type with its methods, then the utility
functions, each group in the order of first use. The behavior does not
change.

Issue: 947bddb