Add ability to tag users

[Bisaloo]

This is not very straightforward. My understanding is that you cannot tag people directly using their username. You have to use their user ID instead. And the user ID cannot be guessed, we have to get it from slack itself.

We could either:

• maintain a dictionary of names <-> user IDs
• get the user ID from the slack API using the fact that everybody joined with their LSHTM email address. We would just have to include the full names in the google doc (or otherwise have a dictionary first name <-> full name but we're better off with the first solution then)

Resources: https://api.slack.com/reference/surfaces/formatting#mentioning-users

[Bisaloo]

I'm going with getting user ID from email addresses with this endpoint: https://api.slack.com/methods/users.list.

As noted there, the bot needs the users:read and users:read.email permissions. I've updated the bot accordingly. I don't think this is a privacy issue since as mentioned above, people joined with their LSHTM address, which is already public/known to anyone with access to the bot.

@sbfnk, the new permissions will be available after the bot is reinstalled. Could you do this whenever convenient please?

[Bisaloo]

https://github.com/epiforecasts/slack_bot/commit/e9ff70cc54a4410b7a10c51ca190e9922e99a601