Show message formatting tips

[preetmishra]

The web app has a section under keyboard shortcuts that shows a list of message formatting help tips. This, in particular, is helpful for users that are relatively new to markdown syntax.

It would be good to make it context-dependent.

Conversation link: https://chat.zulip.org/#narrow/stream/206-zulip-terminal/topic/Message.20formatting.20tips

Screenshot:

[preetmishra]

@zulipbot add "enhancement"

[vinitwadgaonkar]

@zulipbot claim

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