Fix escaping for Slack

[jburianek]

Slack doesn't want all HTML entities escaped, only &, <, and >.
https://api.slack.com/reference/surfaces/formatting#escaping

You shouldn't HTML entity-encode the entire text, as only the specific characters shown above will be decoded for display in Slack.

But marked seems to assume we want HTML output.

To work around this, override the tokenizer's inlineText method with one that only escapes those 3 characters.

This fixes the issue where We've allocated displays as We've allocated in Slack.

[jburianek]

cc: @marissamarym @EndBug

[marissamarym]

Thanks @jburianek!