Comment reactions should be exposed via REST API

Reactions to comments (such as "thumbs up") are not accessible via the REST API for JIRA. They're also not included in the `renderedBody` HTML.

Having reactions exposed in the API would give us the tools to implement a feature to inform users where their actions are required to follow up on issues. Such as:

• when a user is mentioned in a comment of an issue
• and they haven't reacted with a newer comment
• or they haven't reacted with a "reaction" on that comment
• then inform them

Discovering such issues (and also confluence inline comments) is a big bottleneck in communications in all companies I've worked with. I'm sure we're not the only ones who would benefit. Exposing reactions in the API would be a first step to enable such features.