[WIP] Change Welcome Bot message interactions

[alya]

DMs with the Welcome Bot are a good space for playing with Zulip's messaging features, but the commands it offers don't seem super helpful at this point. To reduce the emphasis on commands, and the annoyance from Welcome Bot's replies when it doesn't understand the message, we should:

1. Drop this part from Welcome Bot's original message:

I can also help you get set up! Just click anywhere on this message or press r to reply.

Here are a few messages I understand: apps, profile, theme, channels, topics, message formatting, keyboard shortcuts, help.

2. The first time a user sends a message that Welcome Bot doesn't understand, reply with:

You can chat with me as much as you like! To get help, try one of the following messages: `apps`, `profile`, `theme`, `channels`, `topics`, `message formatting`, `keyboard shortcuts`, `help`.  

3. Do not send a reply to any future message that Welcome Bot doesn't understand. We can still implement some version of Add emoji reactions to Welcome Bot #20023, to have Welcome Bot acknowledge messages it receives.

CZO thread

[zulipbot]

Hello @zulip/server-bots, @zulip/server-onboarding members, this issue was labeled with the "area: onboarding", "area: bots" labels, so you may want to check it out!