#18 Simplify SessionGroups and SessionMessages for basic implementation

[steppy452]

PR Checklist

Please check if your PR fulfills the following requirements:

• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[x] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[x] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

Previously, it was necessary to always manually loop through Sessions and Conversations, even if you were just using the included components from Reachat.

For example,

<SessionGroups>
  {groups =>
    groups.map(({ heading, sessions }) => (
      <SessionsGroup heading={heading} key={heading}>
        {sessions.map(s => (
          <SessionListItem key={s.id} session={s} />
        ))}
      </SessionsGroup>
    ))
  }
</SessionGroups>

<SessionMessages>
  {conversations =>
    conversations.map((conversation, index) => (
      <SessionMessage
        key={conversation.id}
        conversation={conversation}
        isLast={index === conversations.length - 1}
      />
    ))
  }
</SessionMessages>

Issue Number: #18

What is the new behavior?

No longer need to manually loop through Sessions and Conversations. Now, it's only necessary if user wants to use custom designed components.

New API looks like this, though old way of doing it still works:

<SessionGroups />
<SessionMessages />

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

Also, fixed the custom component story that broke when <MessageFiles /> was moved into <MessageQuestion />

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[amcdnl]

We need to update the docs on the site too.